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    My VERY first tube amp.

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    Tevian

    Posts : 7
    Join date : 2014-08-17

    My VERY first tube amp.

    Post by Tevian on Tue Aug 26, 2014 6:02 am

    Hello every one. As the title says this is my VERY first tube amp. To my knowledge I haven't actually sat and listened to any modern tube amps except for maybe a guitar amp here and there.

    So I spotted a reasonably priced old ST-70 amp on craigslist for $400. The amp worked but had some issues. 90% original wiring and someone kinda replaced the output terminals. Lots of heat sweat around the old 7199 input tubes. It did work but had a bit of hum and noise, but had a new full set of output tubes and rectifier. He shaved a bit off the price and I took it home. I'm not totally sure what and manufacture date is, but it doesn't have super old looking wire from the transformers. I'm guessing early 70's but I could be wrong.

    I had come across this amp in the passed and was aware of the modification that were popular. I was just lucky that I had the cash and was willing to give this whole tube thing a shot. After digging into the amp I decided to just replace everything.

    New quad cap, octal driver board, sockets, input terminals and so on. It took two nights and got everything right the first power up. Its now singing sweet music.

    First impression:

    I knew tubes get hot but was a little weary of just how hot these thing get. At full warm up I'm measuring near 400 degrees F on the power tubes. Wow that's hot.

    The amp with old board and parts had lots of noise, but after the rebuild there is none. I mean almost zero noise from the amp. I have a few other solid state amps and they all have some noise but this has none that I can detect with my ear. Of course this is very dependent on the source.

    The amp currently has JJ EL34's. They are brand new and will give this amp some time before I settle on whether it sounds pleasing or not. I'll definitely give the amp some one-on-one time. Here are some pics the enjoy.
       
    Update:

    I purchased some JJ KT-77's from a local dealer and i'm now running them. I let them warm up good them re-biased them. The EL-34's were dead quiet but the left channel now has a very quiet hum that has faded over the last few days. Is this a normal thing for brand new tubes? Like I mentioned before the EL-34's don't sound unpleasant. The amp overall had a very heavy mid-range at first impression. Big difference from my SS amps. My ears are still very new to the tube sound. The KT-77's out of the box have a slightly subdued mid-range compared to the 34's. I pulled out my testing mic and sure enough there was a very slight reduction in the 1k to 3k range. Both treble and bass seemed to preform the same.

    I've read about tube rolling and have seem discussions about which tubes will make a bigger difference on the overall sound profile. Do you guys think replacing the 6SN7's will change more than replacing the power tubes? What i've read is that some of the older tubes may not run at the B+ voltage that is on my new board. What more do I need to know before plugging in random preamp tubes? Also my power transformer runs rather hot. I've read on the forum that the original was a bit small for the amp. Do you guys think its a "must replace" item? Whats the down side of just letting it run a little hot. It hovers around the 140's F but takes about 40 minutes to get that hot.

    Assembling the octal board was a snap and the documentation was very useful. However I did receive a small print out of a different wire diagram for the 2.2k resistor that differs from the schematics that I can find. As you can see in the picture below I wired it as the supplement showed. The B+ hovers near 380v DC.

    Assuming I did a good job I hope this amp will provide plenty of listening enjoyment. Haven't made my mind yet on whether it sounds better but it definitely has a "different but not unpleasant" sound.










    zx

    Posts : 194
    Join date : 2011-08-05

    Re: My VERY first tube amp.

    Post by zx on Tue Aug 26, 2014 7:54 am

    Thank for your input.. an pic....sweet...only down side i have ever found with the 70 is i need more power....so the MK3s are for me............never like the jj tubes....any of them...but thats me
    like the 6sn7 tone...caps can make a big diff in the sound also......
    As for the power trany runing hot....i seen them get vary hot ....an only last 50-60years....hehe....the 6sn7 well draw more than the stock 7199 inputs on B+.... go with SS diodes an pull the 5ar4 less stress on B+ an hearters....less heat!



    Have fun with tubes....there not for everyone....but diff tubes can make them sound better.......


    Thanks for the site Bob....................

    corndog71

    Posts : 454
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    Location : It can get windy here

    Re: My VERY first tube amp.

    Post by corndog71 on Tue Aug 26, 2014 10:08 am

    Looks great! Nice pictures.

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: My VERY first tube amp.

    Post by Jim McShane on Tue Aug 26, 2014 10:25 am

    I can't be 100% sure from the pictures, but it appears the plates of the EL34s have some red color showing in the crease. You can see it best on the 2nd from the bottom picture - look at the tube on the furthest right.

    That can indicate either a problem with the tube, or the tubes are not biased properly. It certainly indicates the tubes are running hotter than is needed (more than likely).
    You did adjust the bias when you replaced the tubes and did the other work? You might want to recheck it even if you did, and readjust it again.

    FYI - I agree with the other poster that JJ tubes are not a good choice. I don't even carry any octal base JJs due to poor quality issues.

    Guest
    Guest

    Re: My VERY first tube amp.

    Post by Guest on Tue Aug 26, 2014 10:27 am

    Nice looking amp!
    My only recommendation would be for you to consider upgrading the power transformer to the one used in the Tubes4Hifi ST70. Since you are running the octal VTA driver and the 6SN7's draw somewhat more current, the stock transformer is being pushed to its limits.
    Oh, and welcome to the abyss of the tube world, once you enter, you will never leave!! Laughing

    Guest
    Guest

    Re: My VERY first tube amp.

    Post by Guest on Tue Aug 26, 2014 10:50 am

    one thing I noticed in your build, you are using crimps for some of the wire extensions.
    Personally, I would not use those, I always solder extensions. The reason is 'mechanical' connection versus solder, solder is much more reliable, provided it is done correctly, especially in high voltage and high current areas in your amp. I recommend you solder them and use heat shrink to insulate the joints.

    Bob Latino
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    Location : Massachusetts

    Re: My VERY first tube amp.

    Post by Bob Latino on Tue Aug 26, 2014 11:59 am

    I agree with what Montana Way said in post #5. The PA-060 power transformer on the original Dynaco ST-70 was too small for the amp to begin with and ran hot. By running three octal driver tubes instead of the noval (9 pin) VTA driver board, you are making the amp run even hotter. Jim McShane would know but I am guessing that an octal 6SN7 tube uses about double the current of a 12AU7 driver tube ... maybe .60 amps for a 6SN7 and .30 amps for a 12AU7. If you multiply that by three tubes, you get over 1 extra amp of current draw over the use of three 12AU7's.

    If you are going to play that amp on a daily basis, I would, as Montana Way mentioned, consider changing to one of the newer upgraded PA-060 power transformers from Dynakitparts.

    Bob

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: My VERY first tube amp.

    Post by Jim McShane on Tue Aug 26, 2014 1:58 pm

    You are correct Dr. Latino! 600 ma. is the heater draw on 6SN7s, that's twice a 12AU7's draw.

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: My VERY first tube amp.

    Post by stewdan on Tue Aug 26, 2014 8:07 pm

    Question for Bob since I am a little confused --

    As I remember it from the Dynaco Literature of the late 1950s and 1960s, the ST70/ST35 and the Mono Amps (Mark II/III/IV) were originally designed to use the PAM-1 preamps plugged into their front panels.  

    And since each PAM-1 used two 12AX7 tubes, is that not about an extra 0.8 to 1.0 Amp of available current for use by the ST70??  Admittedly, we might be on the ragged edge of current availability??!!

    I ask, because I took what was a stock 1960's ST70 and used one of Roy's Octal boards along with an 80-40-30-20 Quad Cap to produce what I hear as a better sounding amp.  The transformers seem to run a little warmer/hotter, but the amp sounds great!!!

    So. I guess my question is --- don't we have a push, since the current draw from the PAM-1's is available???

    Stew

    Bob Latino
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    Re: My VERY first tube amp.

    Post by Bob Latino on Tue Aug 26, 2014 9:13 pm

    stewdan wrote:Question for Bob since I am a little confused --

    As I remember it from the Dynaco Literature of the late 1950s and 1960s, the ST70/ST35 and the Mono Amps (Mark II/III/IV) were originally designed to use the PAM-1 preamps plugged into their front panels.  

    And since each PAM-1 used two 12AX7 tubes, is that not about an extra 0.8 to 1.0 Amp of available current for use by the ST70??  Admittedly, we might be on the ragged edge of current availability??!!

    I ask, because I took what was a stock 1960's ST70 and used one of Roy's Octal boards along with an 80-40-30-20 Quad Cap to produce what I hear as a better sounding amp.  The transformers seem to run a little warmer/hotter, but the amp sounds great!!!

    So. I guess my question is --- don't we have a push, since the current draw from the PAM-1's is available???

    Stew

    Hi Stew,

    Almost no one ever used the PAM-1's (introduced in 1957) with the octal umbilical cord with the ST-70 (introduced in 1959). The PAS-2 stereo preamp came out in 1960 and no one used the PAM-1's after that. Sure, the Stereo 70 was made to work with the PAM-1 but the ST-70 was underpowered even with the original PA-060 using just two 7199 driver tubes. Don't get me wrong .. I love older Dynaco tube gear but all the original Dynaco products were designed to sell at a price point and the "bean counters" at Dynaco skimped here and there to save a nickel here and a dime there to keep the price down. When the ST-70 KIT came out in 1959 it sold for $99.95 with a complete tube set and a tube cage. It still amazes that, at times, stores would DISCOUNT the kit to $89.95!

    When I approached my transformer company to produce the power transformer for the VTA ST-70 amp kit I gave them the specs that I wanted .. Two 6.3 volt @ 5 amp filament lines, a 4 amp rectifier line, 375 milliamp current flow capability etc, etc. I also gave them the cut out size (3 3/4 inches by 3 inches) and had THEM figure the stack height needed. They told me a 2 1/8 inch stack is needed to support that amount of current flow. The stack on the VTA ST-70's power transformer is about 41% higher than the original PA-060 which has a 1 1/2 inch stack (Note - I have also seen 1 5/8 inch stacks on some original Dynaco ST-70's).

    I am not saying that an original PA-060 will fail absolutely if you use an octal driver board. I am saying that in light of the fact that the power transformer is underpowered to begin with, is probably over 50 years old and runs hot, you would probably be better off (IF you use this amp on a daily basis as your main amp) to buy an upgraded power transformer.

    Bob

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: My VERY first tube amp.

    Post by stewdan on Tue Aug 26, 2014 9:42 pm

    Hi Bob -- Thanks for clarifying!!

    And you are right about companies discounting the Dynaco products.  I remember paying $175 (total) in 1965 for factory built ST70 and PAS3X units from EJ Korvette which was a discount Department Store in New York City.

    Stew

    Tevian

    Posts : 7
    Join date : 2014-08-17

    Re: My VERY first tube amp.

    Post by Tevian on Wed Aug 27, 2014 5:24 am

    Great info guy!

    Jim, I do see the hot spot on the far right tube. The biasing is good at full temp. Is that hot spot going to be a big problem? I realize the JJ's are not top shelf, but that's what I had access to. Down the road I would like to try different tubes both power and preamp.

    So I was doing some reading about filament heater supply. I quickly discovered the debate about AC vs DC filament supply. It appears I can buy supplemental filament heater transformers. At the cost savings of buying a full new HVT would it be advisable to simply add a heater transformer for the octal driver tubes and cut them off of the HVT. This would somewhat reduce the current draw and allow me to possibly try tubes with slightly higher heater current.

    What do you think?

    Thx for the info again. So far the amp is doing well. I'm getting in a newer cartridge for my record player and will be listening to some vinyl come this weekend. I will also try changing the caps and see if that changes anything.

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: My VERY first tube amp.

    Post by zx on Wed Aug 27, 2014 7:56 am

    Just so you get my point on the jj tubes .....theres a lot of EL34s that sale for same $$
    I dislike the jj tubes be case there sound suckes....Too me....
    An i cant say that about minny other tubes......others can speak for there selfs....an well...hehe
    An yes the caps can make are brake the sound of any preamp,amp tube are SS...i have found
    As for the power tran....the 5ar4 pulls a lot of current..... i would try two diodes from ratshak 1kV 3amps.... for like $3 bucks......
    Also if you have a Vairc....lower the Ac V coming in.. an the amp well sound better an this well take the some of the stress off the tran.....if your Ac is like most to day... its high...from what was when these amps were made in the 60-70s....
    I use SS diodes in some of my tube preamps.. tube amps an the sound can be sweet.....its your call...sound wise.
    you can get a lot of time ... an good tube sound out of your 70 just like it is....
    In the end tube sound my not be for you ...but you well know.....
    have fun with tubes....hehe
    I have for over 50 years....but i have SS also....an i have got SS to sound more tube like.....


    Thanks for the site Bob.........

    ..

    corndog71

    Posts : 454
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    Location : It can get windy here

    Re: My VERY first tube amp.

    Post by corndog71 on Wed Aug 27, 2014 10:16 am

    Bob Latino wrote:When the ST-70 KIT came out in 1959 it sold for $99.95 with a complete tube set and a tube cage. It still amazes that, at times, stores would DISCOUNT the kit to $89.95!


    According to an online inflation calculator that $99 in 1959 comes out to $810 in today's currency!

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: My VERY first tube amp.

    Post by Jim McShane on Wed Aug 27, 2014 4:25 pm

    Tevian wrote:Great info guy!

    Jim, I do see the hot spot on the far right tube. The biasing is good at full temp. Is that hot spot going to be a big problem? I realize the JJ's are not top shelf, but that's what I had access to. Down the road I would like to try different tubes both power and preamp.

    So I was doing some reading about filament heater supply. I quickly discovered the debate about AC vs DC filament supply. It appears I can buy supplemental filament heater transformers. At the cost savings of buying a full new HVT would it be advisable to simply add a heater transformer for the octal driver tubes and cut them off of the HVT. This would somewhat reduce the current draw and allow me to possibly try tubes with slightly higher heater current.

    What do you think?

    Thx for the info again. So far the amp is doing well. I'm getting in a newer cartridge for my record player and will be listening to some vinyl come this weekend. I will also try changing the caps and see if that changes anything.

    I'm glad I could help!  Very Happy

    It would be wise to measure the plate and screen dissipation just to be safe. The cause of that red is likely the screen is misaligned in the tube, but you need to be sure the wattage dissipated by the plate and screen is within spec. If you don't know how to measure that post again in this thread - but do a Google search first, there's a lot of info out there.

    Yes, removing the heater load from the main power trafo and using supplemental trafos works fine. BUT - the original power trafo is 50 years old, so if it was me I'd take Bob's advice about getting a replacement. Believe me, you'll never look back, it's a significant improvement. It covers all your concerns.

    Tevian

    Posts : 7
    Join date : 2014-08-17

    Re: My VERY first tube amp.

    Post by Tevian on Thu Aug 28, 2014 5:35 am

    So i'm not hearing a lot of love for the old tranny.  tongue  I guess i'll just bite the bullet and cough up the money.  Rolling Eyes

    XY, I understand the rectifier pulls big current but I just like having the extra tube. It just looks cool. Cool You mention using a variac to lower the mains. If I do replace the HVT will I still have to worry about the mains voltage?

    Thanks for the advise Jim. When I get a chance i'll flip it and start measuring the voltages.

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: My VERY first tube amp.

    Post by zx on Thu Aug 28, 2014 8:28 am

    The Varic well help with tube life an sound... people here talk of tube death..
    i have newer tube amps here.... that are made to work with todays AC 120-125 ...but i find that all sound better with less B+ an less heater V...
    After haveing minny hhscott tube amps an preamps that run there tubes heaters at about 5.7-6v an thay give some of the best sound i have ever got out of tubes..for my ears a littel less V can be more....an it been years an NO tube death for me....new tube are NOS ....justsaying.....
    An as far as the new tran..as i said i like more power than the ST70....an thats were i would put my money....
    But if it more about looks for you....an thats cool......it only money....hehe....your money, your call.....
    This is all just one mans finding..............have fun with tubes..



    Thanks for the site Bob..............

    sKiZo

    Posts : 1313
    Join date : 2013-04-01
    Location : Michigan USA

    Re: My VERY first tube amp.

    Post by sKiZo on Thu Aug 28, 2014 1:05 pm

    Tevian wrote:I understand the rectifier pulls big current but I just like having the extra tube. It just looks cool.

    Yup ... I'm a fan of the glow too. Problem being, the 5AR4 handles the EL34's well if you don't drive them too hard, but is borderline at best once you start bumping up on the power. Here's what happens to a Chinese 5AR4 if you try driving a quad of KT120's ...



    Keep an eye out for an old Mullard GZ34 - same ratings, but much more robust than today's production. Maybe even a GZ33 or GZ37, but those will cost you. Both of those are basically the equivalent of two 5AR4s, but the GZ37 can handle more plate current than the GZ33.

    Oh. And just for controversy's sake ... bottles sound better! geek

    Jim McShane

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    Re: My VERY first tube amp.

    Post by Jim McShane on Thu Aug 28, 2014 7:31 pm

    "Keep an eye out for an old Mullard GZ34 - same ratings, but much more robust than today's production."


    I'm not so sure that's true - or as true as it once was. The new production Genalex and Tung-Sol have been bulletproof. Only time will tell, but IMHO you don't have to pay for old stock anymore to get a high quality and durable rectifier tube.


    Last edited by Jim McShane on Fri Aug 29, 2014 5:34 pm; edited 1 time in total

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    Re: My VERY first tube amp.

    Post by Guest on Fri Aug 29, 2014 11:01 am

    I know a lot of people prefer the look of glass and the glow....but it is near impossible to improve upon a Weber Copper TopWZ34 or WZ68....those things are 88mm flak proof!!!!!!!!

    deepee99

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    Re: My VERY first tube amp.

    Post by deepee99 on Fri Aug 29, 2014 2:34 pm

    MontanaWay wrote:I know a lot of people prefer the look of glass and the glow....but it is near impossible to improve upon a Weber Copper TopWZ34 or WZ68....those things are 88mm flak proof!!!!!!!!

    I live with the copper-caps, too, but Holger I gotta tell ya when you light up a pair of Mullard GZ-33s in the middle of winter, the house lights go dim and the dog gets out of your lap 'cuz he doesn't need the heat anymore and the bass plumps up just ever so nice. Just don't ever blow one up; they're about $150 to replace.

    tubes4hifi
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    Re: My VERY first tube amp.

    Post by tubes4hifi on Fri Aug 29, 2014 3:35 pm

    nice photoshop sKiZo !

    deepee99

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    Location : Wallace, Idaho

    Re: My VERY first tube amp.

    Post by deepee99 on Fri Aug 29, 2014 3:42 pm

    tubes4hifi wrote:nice photoshop sKiZo !
    Yeah, SKiz blew it there. Note the position of the ammeters. Or was this after the fuse failed?

    deepee99

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    Re: My VERY first tube amp.

    Post by deepee99 on Fri Aug 29, 2014 3:44 pm

    Jim McShane wrote:"Keep an eye out for an old Mullard GZ34 - same ratings, but much more robust than today's production."


    I'm not so sure that's true - or as true as it once was. The new production Genalex and Tung-Sol have been bulletprof. Only time will tell, but IMHO you don't have to pay for old stock anymore to get a high quality and durable rectifier tube.

    Jim, anyone making a decent GZ-33 anymore? I hate the cost of those NOS Mullards.

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    Re: My VERY first tube amp.

    Post by Guest on Fri Aug 29, 2014 3:55 pm

    deepee99 wrote:
    MontanaWay wrote:I know a lot of people prefer the look of glass and the glow....but it is near impossible to improve upon a Weber Copper TopWZ34 or WZ68....those things are 88mm flak proof!!!!!!!!

    I live with the copper-caps, too, but Holger I gotta tell ya when you light up a pair of Mullard GZ-33s in the middle of winter, the house lights go dim and the dog gets out of your lap 'cuz he doesn't need the heat anymore and the bass plumps up just ever so nice. Just don't ever blow one up; they're about $150 to replace.

    oh no doubt!....but at that cost....looking pretty aint worth that much!!! Shocked

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