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Dogstar

Peter W.

deepee99

GreggW

8 posters

EL34 vs 6550

GreggW: Posts : 80
Join date : 2015-07-15

Post n°1

EL34 vs 6550

by GreggW Sat Aug 03, 2019 12:48 pm

Had a chance to do an unscientific AB comparison with my son's ST70 and my ST120 using a fully loaded PAS3 (yes, it's really not a PAS3 any more) with tone controls straight up. I think heard a little more bass out of the Valve Art EL34 than the EH6550. Speakers are 80's Infinity RS6. I didn't dime the volume but played them pretty loud and soft. Just wondering if my perception is reality. Both amps were built within a year of each other and have the 12AT7 driver boards, same tubes, with upgraded coupling caps.

For the experienced tube rollers, do the Gold Lion KT88s reach a little deeper than the EH6550?

Guest: Guest

Post n°2

Re: EL34 vs 6550

by Guest Sat Aug 03, 2019 5:46 pm

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Last edited by PeterCapo on Mon Nov 30, 2020 5:56 pm; edited 1 time in total

deepee99: Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°3

Re: EL34 vs 6550

by deepee99 Mon Aug 05, 2019 12:53 pm

Can't speak to the Russian and Chinese tubes, but NOS KT-88s are a tad boomier in the base than are the old TS6550s. At least in my ST-120 that's the case.

Peter W.: Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA

Post n°4

Re: EL34 vs 6550

by Peter W. Mon Aug 05, 2019 1:34 pm

A few things, in no particular order:

a) The VTA 70 and 120, I believe, have Damping Factors around 16.
b) Which "ain't nohow very much".
c) Meaning that "boominess" will be in direct proportion to overall Frequency Response, not power, directly.
d) Keep in mind that the difference between 35 watts and 60 watts is less than 3 dB.
e) Which, again, "ain't nohow very much".

The deeper any tube type or brand reaches in frequency will be the single and singular factor in this discussion.

Dogstar: Posts : 361
Join date : 2014-06-23

Post n°5

Re: EL34 vs 6550

by Dogstar Tue Aug 06, 2019 11:00 am

I distinctly noticed KT120’s having more putting out more bass than KT88’s. And that was with either Sovtek or Genelex KT88’s compared to TungSol KT120’s.

I understand the comparison was between EL34’s and 6550’s.

It was pointed out to me a long time ago that what a lot of people perceive to be a deep bass response was actually the sound of the lower frequencies being muddier rather than a more accurate sound output of the various musical instruments that are associated with deep low frequencies sounds such as a tuba.

Electronically synthesized sounds coming from a keyboard or bass guitar can also be included but then your considering music that may not be complimented by a tube system.

Guest: Guest

Post n°6

Re: EL34 vs 6550

by Guest Tue Aug 06, 2019 11:12 am

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Last edited by PeterCapo on Mon Nov 30, 2020 4:27 pm; edited 1 time in total

DonP: Posts : 8
Join date : 2019-05-30

Post n°7

Re: EL34 vs 6550

by DonP Fri Aug 09, 2019 7:35 am

I have a bunch of GE 6550A's and an ST-70 that needs better tubes. Wondering if I can be pointed in the direction of recommended changes to the ST-70 in order to safely run the 6550s.

I know I need to crank up the bias to keep the current under control.

I have the smaller 1 1/2" plate stack power transformer that already runs warm. So I may be looking at a new PT.

New screen resistors? Grid resistors?

Any help appreciated.

Thanks.

Peter W.: Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA

Post n°8

Re: EL34 vs 6550

by Peter W. Fri Aug 09, 2019 7:47 am

DonP wrote:

I have a bunch of GE 6550A's and an ST-70 that needs better tubes. Wondering if I can be pointed in the direction of recommended changes to the ST-70 in order to safely run the 6550s.

I have the smaller 1 1/2" plate stack power transformer that already runs warm. So I may be looking at a new PT.

Short answer:

Just don't. An OEM 70 is simply not 'up' to 6550s under any circumstances.

Best option is either one of Bob's 120s, or a pair of his monoblocks.

DonP: Posts : 8
Join date : 2019-05-30

Post n°9

Re: EL34 vs 6550

by DonP Fri Aug 09, 2019 8:19 am

Peter W. wrote:

DonP wrote:

I have a bunch of GE 6550A's and an ST-70 that needs better tubes. Wondering if I can be pointed in the direction of recommended changes to the ST-70 in order to safely run the 6550s.

I have the smaller 1 1/2" plate stack power transformer that already runs warm. So I may be looking at a new PT.

Short answer:

Just don't. An OEM 70 is simply not 'up' to 6550s under any circumstances.

Best option is either one of Bob's 120s, or a pair of his monoblocks.

Thanks. I guess I need to find some EL34's then. My ST-70 came with the Russian 6L6's (6P3-E or something like that). It works great, but this is my only big tube hi-fi amp, and I'd like tubes that are matched better to the impedance of the OT's.

Looks like I'll have to do a VTA-120 build as well.

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°10

Re: EL34 vs 6550

by Bob Latino Fri Aug 09, 2019 8:29 am

My opinion on this below ...

1. Don't run 6550, KT88 or KT120 output tubes in any STOCK Dynaco ST-70. The stock PA-060 power transformer is "weak" in original Dynaco ST-70's and tends to run hot to begin with. The use of these tubes will cause an original Dynaco ST-70 to run even hotter and could damage the 43 to 60 year old PA-060 power transformer on these amps. If you pull the power transformer and replace it with a new Dynakitparts PA-060, you can run 6550 or KT88 output tubes in your original Dynaco ST-70. I would not run KT120 output tubes in any "ST-70" amp.

2. In order to run 6550 and/or KT88 in an original ST-70 (after you replaced the power transformer) you have to alter the original amps bias system's range on the 7 lug terminal strip in order to crank in more negative DC voltage to control the 6550 or KT88 output tubes. You would do this by dropping the value of the two 10K resistors on the 7 lug terminal strip. One 10K is on lugs 1 and 2 and the other 10K is on lugs 3 and 4. I don't know what actual value these resistors should be but you could try two 5K resistors. You should run 6550 or KT88's at 50 milliamps each which on the original Dynaco bias system is 1.56 VDC.

3. In order to run 6550 or KT88 in a VTA ST-70, all you have to do is drop the value of R39 from 7.5K to about 3.5K. On the VTA ST-70, R39 is on the BOTTOM of the driver board between the two 100 uF @100 volt capacitors. Just unsolder that resistor and put in the smaller (1/2 watt) resistor.

4. Summary > In any Dynaco or VTA ST-70 you can use 6L6, EL34, 6CA7, KT66 or KT77 output tubes. If you have an upgraded power transformer on your Dynaco ST-70 or you have a VTA ST-70, you can also use 6550 or KT88 output tubes IF you modify the bias system with resistor changes.

Bob

DavidR and DougWHolt like this post

deepee99: Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°11

Re: EL34 vs 6550

by deepee99 Fri Aug 09, 2019 8:50 am

I'd become quite taken with the El-34 in my VTA ST-70, ANOS Mullards. I also had in stock a quad of NOS Siemens, so on a lark and after asking around, swapped out the antique British KT-88s in my ST-120 for the El-34s and do my delight they sound (to me) every bit as good, or even a tad sweeter. Driving a load of 91dB Highland H-1s (Tyler Acoustics) at above-medium and into the red zone SPLs exhibits no sign of strain on the tubes' part -- I might just keep them in there and unload my expensive KT-88s, though I will keep one quad of virgin Tung Sol 6550s just on principle.
At any rate, my amps, my speaks, my room, my ears, all subjective in and of themselves, unless you're trying to drive Maggies or their ilk, the El-34 is a fine substitute for its bottle-shaped cousins. El-34s used to be dirt-cheap, but now instead of $100-$200 a quad they're fetching $500 on up, so I'm glad I got mine when I did.
My ST-120's nominal bias is 0.375 VDC as determined by Pavel's autobias, which gives a little headroom for the Ei-34s, which max out at 0.40 VDC. So no change in bias setting from the larger tubes.

DonP: Posts : 8
Join date : 2019-05-30

Post n°12

Re: EL34 vs 6550

by DonP Sat Aug 10, 2019 4:40 pm

Bob Latino wrote: My opinion on this below ...

1. Don't run 6550, KT88 or KT120 output tubes in any STOCK Dynaco ST-70. The stock PA-060 power transformer is "weak" in original Dynaco ST-70's and tends to run hot to begin with. The use of these tubes will cause an original Dynaco ST-70 to run even hotter and could damage the 43 to 60 year old PA-060 power transformer on these amps. If you pull the power transformer and replace it with a new Dynakitparts PA-060, you can run 6550 or KT88 output tubes in your original Dynaco ST-70. I would not run KT120 output tubes in any "ST-70" amp.

2. In order to run 6550 and/or KT88 in an original ST-70 (after you replaced the power transformer) you have to alter the original amps bias system's range on the 7 lug terminal strip in order to crank in more negative DC voltage to control the 6550 or KT88 output tubes. You would do this by dropping the value of the two 10K resistors on the 7 lug terminal strip. One 10K is on lugs 1 and 2 and the other 10K is on lugs 3 and 4. I don't know what actual value these resistors should be but you could try two 5K resistors. You should run 6550 or KT88's at 50 milliamps each which on the original Dynaco bias system is 1.56 VDC.

3. In order to run 6550 or KT88 in a VTA ST-70, all you have to do is drop the value of R39 from 7.5K to about 3.5K. On the VTA ST-70, R39 is on the BOTTOM of the driver board between the two 100 uF @100 volt capacitors. Just unsolder that resistor and put in the smaller (1/2 watt) resistor.

4. Summary > In any Dynaco or VTA ST-70 you can use 6L6, EL34, 6CA7, KT66 or KT77 output tubes. If you have an upgraded power transformer on your Dynaco ST-70 or you have a VTA ST-70, you can also use 6550 or KT88 output tubes IF you modify the bias system with resistor changes.

Bob

Thanks Bob. I'll hold off until I can purchase a VTA-120 for the 6550s. I've done the mod to Marshall 100's (bias cicuit changes to go EL34->6550), but I want to keep the ST-70 as is. if I upgrade the PT, then I have an extra PT laying around to do something with. It becomes an endless cycle.

I'll try tracking down a quad of EL34's for now.

deepee99: Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°13

Re: EL34 vs 6550

by deepee99 Sat Aug 10, 2019 6:32 pm

DonP wrote:

Bob Latino wrote: My opinion on this below ...

1. Don't run 6550, KT88 or KT120 output tubes in any STOCK Dynaco ST-70. The stock PA-060 power transformer is "weak" in original Dynaco ST-70's and tends to run hot to begin with. The use of these tubes will cause an original Dynaco ST-70 to run even hotter and could damage the 43 to 60 year old PA-060 power transformer on these amps. If you pull the power transformer and replace it with a new Dynakitparts PA-060, you can run 6550 or KT88 output tubes in your original Dynaco ST-70. I would not run KT120 output tubes in any "ST-70" amp.

2. In order to run 6550 and/or KT88 in an original ST-70 (after you replaced the power transformer) you have to alter the original amps bias system's range on the 7 lug terminal strip in order to crank in more negative DC voltage to control the 6550 or KT88 output tubes. You would do this by dropping the value of the two 10K resistors on the 7 lug terminal strip. One 10K is on lugs 1 and 2 and the other 10K is on lugs 3 and 4. I don't know what actual value these resistors should be but you could try two 5K resistors. You should run 6550 or KT88's at 50 milliamps each which on the original Dynaco bias system is 1.56 VDC.

3. In order to run 6550 or KT88 in a VTA ST-70, all you have to do is drop the value of R39 from 7.5K to about 3.5K. On the VTA ST-70, R39 is on the BOTTOM of the driver board between the two 100 uF @100 volt capacitors. Just unsolder that resistor and put in the smaller (1/2 watt) resistor.

4. Summary > In any Dynaco or VTA ST-70 you can use 6L6, EL34, 6CA7, KT66 or KT77 output tubes. If you have an upgraded power transformer on your Dynaco ST-70 or you have a VTA ST-70, you can also use 6550 or KT88 output tubes IF you modify the bias system with resistor changes.

Bob

Thanks Bob. I'll hold off until I can purchase a VTA-120 for the 6550s. I've done the mod to Marshall 100's (bias cicuit changes to go EL34->6550), but I want to keep the ST-70 as is. if I upgrade the PT, then I have an extra PT laying around to do something with. It becomes an endless cycle.

I'll try tracking down a quad of EL34's for now.

Coupla nice quads on the 'Bay right now. Or just get some of the post blight 6CA7/EL34s for <$100/quad.

DonP: Posts : 8
Join date : 2019-05-30

Post n°14

Re: EL34 vs 6550

by DonP Sat Aug 10, 2019 9:50 pm

deepee99 wrote:Coupla nice quads on the 'Bay right now. Or just get some of the post blight 6CA7/EL34s for <$100/quad.

Can you point me in the right direction? I'm afraid of getting an expensive box of rocks.

I did find a local seller and sent him a message, but if the price gets crazy I'm out.

Other than that, should I look for new JJ's? Tung-Sol's? Shuguang's?

Dave_in_Va: Posts : 443
Join date : 2013-04-02
Location : Mid. VA

Post n°15

Re: EL34 vs 6550

by Dave_in_Va Sat Aug 10, 2019 10:37 pm

If you're not going to go the NOS route, the Valve Art EL 34s that came with my VTA ST 70 were very nearly as nice sounding as the NOS Mullards I settled upon.

Whatever you do, install the EL 34s (whatever brand) and use them for 3-4 months. You really can't make snap decisions on power tubes. Whether its tube break in or ear break in, time makes a difference.

(I have a VTA St 70. No clue about the 120.)

deepee99: Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°16

Re: EL34 vs 6550

by deepee99 Sat Aug 10, 2019 11:09 pm

Dave_in_Va wrote:If you're not going to go the NOS route, the Valve Art EL 34s that came with my VTA ST 70 were very nearly as nice sounding as the NOS Mullards I settled upon.

Whatever you do, install the EL 34s (whatever brand) and use them for 3-4 months. You really can't make snap decisions on power tubes. Whether its tube break in or ear break in, time makes a difference.

(I have a VTA St 70. No clue about the 120.)

Here's a very decently priced quad of Siemens El-34s:
https://www.ebay.com/itm/4x-EL34-power-tubes-RFT-SIEMENS-EL-34/312715877206?hash=item48cf519b56:g:6NkAAOSwY7ZdE27G
I'd jump on them myself tonight if I had any bucks. Don't let the "used" part scare you. There's still likely several thousand hours left on them.
I also see no harm in trying a quad of modern knock-offs first. They're dirt cheap and you could try both Russian and Chinese versions for comparison's sake, following Dave_in_Va's caveat that you give each set a 3- or 4-month tryout before swapping.
Another reason for going the el-cheapo route, too: When you finally install a mint quad of NOS power tubes, you'll appreciate why people are willing to spend serious cabbage on them.
Of the old stock, Amperex Bugle Boys are said to be the finest of the El-34 breed. A bit too rich for my blood. I have Siemens in my ST-120, and Mullards in my ST70. Both rock.
Good luck.
dp

Bob Latino: Admin; Posts : 3262
Join date : 2008-11-26
Location : Massachusetts

Post n°17

Re: EL34 vs 6550

by Bob Latino Sun Aug 11, 2019 8:08 am

Dave_in_Va wrote:If you're not going to go the NOS route, the Valve Art EL 34s that came with my VTA ST 70 were very nearly as nice sounding as the NOS Mullards I settled upon.

The EL34's made by Shuguang (Valve Art) are one of the biggest bargains in an EL34 tube. You can get a matched quad for around $55 - $60 USD. These are presently the EL34 output tubes that I ship to VTA ST-70 amp kit customers that want a matched quad of EL34 output tubes with their kits. I have been using these for 13 years and the failure rate has been near zero. They also have a well balanced sound with no excessive emphasis on any one part of the audio spectrum.

Bob

DonP: Posts : 8
Join date : 2019-05-30

Post n°18

Re: EL34 vs 6550

by DonP Sun Aug 11, 2019 9:08 am

deepee99 wrote:

Dave_in_Va wrote:If you're not going to go the NOS route, the Valve Art EL 34s that came with my VTA ST 70 were very nearly as nice sounding as the NOS Mullards I settled upon.

Whatever you do, install the EL 34s (whatever brand) and use them for 3-4 months. You really can't make snap decisions on power tubes. Whether its tube break in or ear break in, time makes a difference.

(I have a VTA St 70. No clue about the 120.)

Here's a very decently priced quad of Siemens El-34s:
https://www.ebay.com/itm/4x-EL34-power-tubes-RFT-SIEMENS-EL-34/312715877206?hash=item48cf519b56:g:6NkAAOSwY7ZdE27G
I'd jump on them myself tonight if I had any bucks. Don't let the "used" part scare you. There's still likely several thousand hours left on them.
I also see no harm in trying a quad of modern knock-offs first. They're dirt cheap and you could try both Russian and Chinese versions for comparison's sake, following Dave_in_Va's caveat that you give each set a 3- or 4-month tryout before swapping.
Another reason for going the el-cheapo route, too: When you finally install a mint quad of NOS power tubes, you'll appreciate why people are willing to spend serious cabbage on them.
Of the old stock, Amperex Bugle Boys are said to be the finest of the El-34 breed. A bit too rich for my blood. I have Siemens in my ST-120, and Mullards in my ST70. Both rock.
Good luck.
dp

Thanks. I saw this too late - auction already over.

DonP: Posts : 8
Join date : 2019-05-30

Post n°19

Re: EL34 vs 6550

by DonP Sun Aug 11, 2019 9:09 am

Bob Latino wrote:

Dave_in_Va wrote:If you're not going to go the NOS route, the Valve Art EL 34s that came with my VTA ST 70 were very nearly as nice sounding as the NOS Mullards I settled upon.

The EL34's made by Shuguang (Valve Art) are one of the biggest bargains in an EL34 tube. You can get a matched quad for around $55 - $60 USD. These are presently the EL34 output tubes that I ship to VTA ST-70 amp kit customers that want a matched quad of EL34 output tubes with their kits. I have been using these for 13 years and the failure rate has been near zero. They also have a well balanced sound with no excessive emphasis on any one part of the audio spectrum.

Bob

Thanks again Bob. I think these are the ones I'm going to track down.

deepee99: Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°20

Re: EL34 vs 6550

by deepee99 Sun Aug 11, 2019 9:35 am

Here's another good price for a quad, Teslas,
https://www.ebay.com/i/264212589231?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160908110712%26meid%3D345b14974994431498b3f2609a15f879%26pid%3D100677%26rk%3D11%26rkt%3D30%26sd%3D312715877206%26itm%3D264212589231%26pg%3D2385738
Can't speak for these but the old Czech-made Tesla KT-88s are a fine tube.

DonP: Posts : 8
Join date : 2019-05-30

Post n°21

Re: EL34 vs 6550

by DonP Sun Aug 11, 2019 9:53 am

deepee99 wrote:Here's another good price for a quad, Teslas,
https://www.ebay.com/i/264212589231?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160908110712%26meid%3D345b14974994431498b3f2609a15f879%26pid%3D100677%26rk%3D11%26rkt%3D30%26sd%3D312715877206%26itm%3D264212589231%26pg%3D2385738
Can't speak for these but the old Czech-made Tesla KT-88s are a fine tube.

Thanks. I'm running a new quad of Tesla's (bought in 1990 - Groove Tube E34Ls, Hard 9) that I just put into my Marshall. They sound great. Compared to the 1998 Winged C's I used to have in there, they need more negative bias to keep the current down. Don't know what that means. Did the Winged C's have more gain, or did they simply flow less current (plate and cathode farther apart?), or who knows?

This is where I get cold feet though. $120 for 4 used tubes from a foreign country. No idea of what shape they are in. I just can't pull the trigger without more details. If I go with the Shuguangs, they would be from Amplified Parts, so I'd have some backing there.

deepee99: Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°22

Re: EL34 vs 6550

by deepee99 Sun Aug 11, 2019 11:09 am

DonP wrote:

deepee99 wrote:Here's another good price for a quad, Teslas,
https://www.ebay.com/i/264212589231?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160908110712%26meid%3D345b14974994431498b3f2609a15f879%26pid%3D100677%26rk%3D11%26rkt%3D30%26sd%3D312715877206%26itm%3D264212589231%26pg%3D2385738
Can't speak for these but the old Czech-made Tesla KT-88s are a fine tube.

Thanks. I'm running a new quad of Tesla's (bought in 1990 - Groove Tube E34Ls, Hard 9) that I just put into my Marshall. They sound great. Compared to the 1998 Winged C's I used to have in there, they need more negative bias to keep the current down. Don't know what that means. Did the Winged C's have more gain, or did they simply flow less current (plate and cathode farther apart?), or who knows?

This is where I get cold feet though. $120 for 4 used tubes from a foreign country. No idea of what shape they are in. I just can't pull the trigger without more details. If I go with the Shuguangs, they would be from Amplified Parts, so I'd have some backing there.

Old used tubes are always a crap-shoot, especially on Fleabay. The preferred setting is of course to meet face-to-face at a Ham Swap Meet or similar, but by-and-large have had pretty good luck on the auctions sites, particularly Canuck Audio Mart where my experience with both sellers and buyers has been excellent.

DougWHolt: Posts : 2
Join date : 2019-03-01
Age : 54
Location : Mooresboro, NC

Post n°23

Would like to modify my ST-70 to run 6550s

by DougWHolt Sat Jan 23, 2021 12:39 am

Bob Latino wrote: My opinion on this below ...

1. Don't run 6550, KT88 or KT120 output tubes in any STOCK Dynaco ST-70. The stock PA-060 power transformer is "weak" in original Dynaco ST-70's and tends to run hot to begin with. The use of these tubes will cause an original Dynaco ST-70 to run even hotter and could damage the 43 to 60 year old PA-060 power transformer on these amps. If you pull the power transformer and replace it with a new Dynakitparts PA-060, you can run 6550 or KT88 output tubes in your original Dynaco ST-70. I would not run KT120 output tubes in any "ST-70" amp.

2. In order to run 6550 and/or KT88 in an original ST-70 (after you replaced the power transformer) you have to alter the original amps bias system's range on the 7 lug terminal strip in order to crank in more negative DC voltage to control the 6550 or KT88 output tubes. You would do this by dropping the value of the two 10K resistors on the 7 lug terminal strip. One 10K is on lugs 1 and 2 and the other 10K is on lugs 3 and 4. I don't know what actual value these resistors should be but you could try two 5K resistors. You should run 6550 or KT88's at 50 milliamps each which on the original Dynaco bias system is 1.56 VDC.

3. In order to run 6550 or KT88 in a VTA ST-70, all you have to do is drop the value of R39 from 7.5K to about 3.5K. On the VTA ST-70, R39 is on the BOTTOM of the driver board between the two 100 uF @100 volt capacitors. Just unsolder that resistor and put in the smaller (1/2 watt) resistor.

4. Summary > In any Dynaco or VTA ST-70 you can use 6L6, EL34, 6CA7, KT66 or KT77 output tubes. If you have an upgraded power transformer on your Dynaco ST-70 or you have a VTA ST-70, you can also use 6550 or KT88 output tubes IF you modify the bias system with resistor changes.

Bob

I have an ST-70 that I’ve stripped down, sandblasted the rusty chassis and painted a dark charcoal black. I did replace with an upgraded the power transformer with the Triode PA060-s so think I’m set for running power hungry 6550s. I will do as you mentioned start out using the 5k resistors in the bias circuit. What about the 2 bias pots. Should I keep them at 10k or change them out with a different value? I have 25k pots, would they give me a larger range with bias adjustments?

Doug

DonP: Posts : 8
Join date : 2019-05-30

Post n°24

Re: EL34 vs 6550

by DonP Sat Jan 23, 2021 11:35 am

DougWHolt wrote:

I have an ST-70 that I’ve stripped down, sandblasted the rusty chassis and painted a dark charcoal black. I did replace with an upgraded the power transformer with the Triode PA060-s so think I’m set for running power hungry 6550s. I will do as you mentioned start out using the 5k resistors in the bias circuit. What about the 2 bias pots. Should I keep them at 10k or change them out with a different value? I have 25k pots, would they give me a larger range with bias adjustments?

Doug

Don't need to change the bias pots. Just follow the recommendations above.

Update from me: I currently have 2 ST-70's. One stock with EL34's and one I bought modified to use Russian 6L6's.

The GE 6550A's I was sitting on are currently loaded into a guitar amp. Sitting there because I rarely power it up - too loud.

Here is one thing I'd like to ask everyone. The Dynaco ST-70 OT's were designed to work with the impedance of the EL34. I know Marshall's have no issues switch from EL34 to 6550, but what about the sound quality in the ST-70? In a guitar amp no one cares as long as it sounds "good". If you run tubes other than the designed EL34's in an ST-70, will there be an impact to the sound quality?

I've heard that my ST-70 running the Russian 6L6's would only give about 25 watts per channel (don't know if this is true or not). I'm thinking of putting it back to stock.

Thoughts?

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Post n°25

Re: EL34 vs 6550

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