Bob what are the differences between using these two tubes in your amp? Is it just different sound presentation? Does the KT-88 increase the output?
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jrethorst
Bob Latino
rwferr
7 posters
EL 34 v. KT 88
Bob Latino- Admin
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rwferr wrote:Bob what are the differences between using these two tubes in your amp? Is it just different sound presentation? Does the KT-88 increase the output?
Hi Richard,
The difference is really just sound presentation. The EL34 tends to be more "tubey" and the KT88 tends to be a little more linear from top to bottom. The KT88 tends to have a little more output at the frequency extremes. When you have an upgraded transformer, as you do, you can use either tube in your amp. In fact in your amp you can use 6L6, EL34, KT66, KT77, 6550 or KT88 tubes.
KT88's will also last longer in an ST-70. They are designed to look at 475 - 500 volts on pins 3 and 4. In your amp they only see about 425 - 430 volts. The down side is that KT88's cost much more than EL34's - $100 to $200 a quad.
You won't gain any more power by using a KT88 or 6550 tube in an ST-70 but you will gain, as mentioned above, a slightly different sound presentation and longer tube life. To those with ST-70's which have a STOCK PA-060 power transformer > Do not use a KT88 or 6550 tube in your ST-70 amp to replace the EL34's. These two tubes will draw maybe 30% more current from an already over taxed stock power transformer and could bring about the demise of your power transformer. Those with a stock power transformer should try 6L6, KT66 or KT77's as alternate output tubes.
Bob
Last edited by Bob Latino on Sat Feb 14, 2009 9:24 pm; edited 1 time in total
rwferr- Posts : 16
Join date : 2009-01-11
Bob I have seen in some other threads that you prefer the KT77 in your amp to the KT-88 or EL-34. What are the differences to you? I have only used the KT-88 so far.
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rwferr wrote:Bob I have seen in some other threads that you prefer the KT77 in your amp to the KT-88 or EL-34. What are the differences to you? I have only used the KT-88 so far.
Richard,
The JJ KT77's are nice sounding but have quality control issues. Many early samples of the JJ KT77's had smaller than normal sized pins which didn't fit the tube sockets well. I bought a quad about 2 years ago and had to send two tubes of the quad back because of this issue. One tube of the quad actually fell out of the socket (!) when I turned the amp upside down to take the bottom cover off. Now supposedly this issue has been addressed by JJ and the new quads have normal sized pins but there still lurks the possibility of some of the older stock still being on some dealers shelves. Genalex Gold Lion now makes a KT77 tube but I have not heard this tube so in all honesty I cannot comment on its sound.
The ElectroHarmonix KT88's that you have in your amp are great sounding tubes for the money and a real good value.
Right now IMHO the best tubes for the ST-70 either a stock amp or a VTA boarded ST-70 amp like yours are the Genalex Gold Lion KT66's. They are a little on the expensive side - about $140 a quad. I use them in my own personal ST-70 and really like the sound. I bias them at .450 volts DC per tube. The Genalex KT77's are relatively new and are as expensive right now as the KT88's - about $200 a quad. I don't think the extra of $60 for the Gold Lion KT77's or KT88's is really worth it at least in an ST-70 based amp. In a Dynaco Mark III or my ST-120, which are 60 WPC amps and have 475 volts DC on the plates of the Mark III and 495 volts on the plates of the ST-120, you NEED the extra durability provided by a KT88 or 6550 tube. The KT88 and 6550 tubes are designed to "look at" 475+ volts on pin 3 (plate) and pin 4 (ultralinear screen tap). An ST-70 amp only has 410 - 430 volts on pins 3 and 4.
Bob
VTA boarded ST-70 with Genalex Gold Lion KT66 output tubes.
rwferr- Posts : 16
Join date : 2009-01-11
Thanks for the info Bob.
Richard
Richard
jrethorst- Posts : 39
Join date : 2009-02-01
How much would you have to do to the power supply of a Mk 3 to run EL34's instead of 6550/KT88's? Would pentode/triode make any difference?
Bob Latino- Admin
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Personally I wouldn't use EL34's in a Mark III. You COULD do it but IMHO the EL34's would have a short life. The EL34's of today are not designed to look at voltages of 475-500 volts DC on pins 3 and 4 as is found on the Mark III. 6550 or KT88's are a better choice in a Mark III.
Bob
Bob
jrethorst- Posts : 39
Join date : 2009-02-01
Bob Latino wrote: Personally I wouldn't use EL34's in a Mark III. You COULD do it but IMHO the EL34's would have a short life. The EL34's of today are not designed to look at voltages of 475-500 volts DC on pins 3 and 4 as is found on the Mark III. 6550 or KT88's are a better choice in a Mark III.
Bob
Would running the Mark III in triode mode make any difference? Especially, would EL34's last longer than they would in pentode mode?
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If you run EL34's in the triode mode in a Mark III you do take about 475-500 volts off pin # 4. By doing so you *may* increase EL34 tube life a little in the Mark III but again the EL34's of today are not designed to look at that kind of voltage for long periods of time. IMHO forcing an EL34 to look at this kind of voltage will shorten tube life. The precursor to the Mark III, the Dynaco Mark II which came out in 1955, did use EL34 output tubes and ran about 460 - 470 VDC on pins 3 and 4 to get about 50 watts per monoblock. When the Mark III came out in 1957 they upped the B+ high voltage to 475 - 480 VDC but, more importantly, they now recommended (and shipped with each amp) KT88 output tubes.
Bob
Bob
jrethorst- Posts : 39
Join date : 2009-02-01
Running in triode, what kind of tube life could you expect in a Mk3 from a KT88, and from an EL34? Are we talking six years? Six months?
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jrethorst wrote:Running in triode, what kind of tube life could you expect in a Mk3 from a KT88, and from an EL34? Are we talking six years? Six months?
It is hard to determine how much time these tubes will last in terms of months or years because we don't know how much they have been used in this time period. "Hours On" is probably a better yardstick with respect to tube life. From what I have seen in my own tube systems is that most output tubes in Dynaco amps when run in pentode ultralinear mode can provide good sound for maybe 2000-3000 hours of playing time. The tube, if it is well made, may last much longer than that but sound quality starts to degrade in a subtle manner. When you run the tube (KT88 or EL34) in triode you only produce 55-60% of the power in pentode so the tube is not being asked to work as hard. As such, it *should* last longer. As to how much longer an output tube will last when run in triode vs. pentode? > I can't really answer that question because I'm not sure. As to your EL34 vs. KT88 question > a KT88 is more durable and will handle higher output than an EL34 and should last longer than an EL34 in any application where both could be used.
Bob
jrethorst- Posts : 39
Join date : 2009-02-01
What wiring changes are there to run EL34's in a Mk3? In addition, are there other changes in addition to:
https://dynacotubeaudio.forumotion.com/basket-f2/run-your-dynaco-st-70-mark-ii-mark-iii-or-mark-iv-in-triode-mode-photo-t41.htm?highlight=mk3+triode
to run in triode mode?
Thank you.
https://dynacotubeaudio.forumotion.com/basket-f2/run-your-dynaco-st-70-mark-ii-mark-iii-or-mark-iv-in-triode-mode-photo-t41.htm?highlight=mk3+triode
to run in triode mode?
Thank you.
jrethorst- Posts : 39
Join date : 2009-02-01
How much is a little for the bias?
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
Ohm's Law:
V = IR
Voltage at Biaset point = current multiplied by the cathode resistor value.
In a typical stock Mark III the cathode resistor is 11.2 ohms.
If you want 40mA of current in each tube, which others on this list have recommended for current production EL34, that makes a total of 80mA for two tubes.
So:
0.080 x 11.2 = 0.896 volts. Call it 0.9.
Substitute other values of desired idle current and cathode resistor if desired and/or needed. For reference: in the Mark II, Dynaco ran EL34s at 65mA but those were original Mullard tubes that were more rugged and long-lived than current Russian, Eastern Euro and Chinese EL34s. In the Mark III, Dynaco ran 6550s at 70mA.
V = IR
Voltage at Biaset point = current multiplied by the cathode resistor value.
In a typical stock Mark III the cathode resistor is 11.2 ohms.
If you want 40mA of current in each tube, which others on this list have recommended for current production EL34, that makes a total of 80mA for two tubes.
So:
0.080 x 11.2 = 0.896 volts. Call it 0.9.
Substitute other values of desired idle current and cathode resistor if desired and/or needed. For reference: in the Mark II, Dynaco ran EL34s at 65mA but those were original Mullard tubes that were more rugged and long-lived than current Russian, Eastern Euro and Chinese EL34s. In the Mark III, Dynaco ran 6550s at 70mA.
jrethorst- Posts : 39
Join date : 2009-02-01
Thank you.
jrethorst- Posts : 39
Join date : 2009-02-01
Lowering the bias for EL34s as poster GP49 suggests, and rewiring mark 3 amps for triode mode, does anyone have a suggestion for brand of EL34? I'm looking at Svetlana and JJ, but happy to look at others.
Last edited by jrethorst on Fri Mar 26, 2021 1:34 am; edited 1 time in total (Reason for editing : clarification)
dmoorecincy- Posts : 11
Join date : 2020-07-16
Personal experience with the ST-70 running new production Tungsol EL34B and Tungsol 6550. It is definitely a different presentation. The 6550s are cleaner/more open sounding in the mids and tighter on the bottom end. Less color if you will. Don't get me wrong the EL34s are good. A little rounder/fatter/thicker/grungier whatever you want to call it. I can see why some guitar amp guys prefer the EL34s and then push them to distort. I started with the 34s but just prefer the 6550 sound. Although I do keep thinking about rolling some 77s or 66s in to see if I like them better...
corndog71- Posts : 840
Join date : 2013-03-19
Location : It can get windy here
I started with EL34s in an ST70 and Mk IVs. I was always curious to see if I could hear a difference with KT88s. Then I built the VTA120 and used Tung-Sol 6550s. Sounded great both ways.
fedoragent- Posts : 57
Join date : 2013-09-14
Bob,
I know that you've retired, and might not see this, but I have to try. I do have a KT88s in my VTA70 with the stock PT. Could these tubes be run in short intervals and biased on the lower side as to not stress out the PT/OT?
Thanks,
Jon
I know that you've retired, and might not see this, but I have to try. I do have a KT88s in my VTA70 with the stock PT. Could these tubes be run in short intervals and biased on the lower side as to not stress out the PT/OT?
Thanks,
Jon
Bob Latino wrote:rwferr wrote:Bob I have seen in some other threads that you prefer the KT77 in your amp to the KT-88 or EL-34. What are the differences to you? I have only used the KT-88 so far.
Richard,
The JJ KT77's are nice sounding but have quality control issues. Many early samples of the JJ KT77's had smaller than normal sized pins which didn't fit the tube sockets well. I bought a quad about 2 years ago and had to send two tubes of the quad back because of this issue. One tube of the quad actually fell out of the socket (!) when I turned the amp upside down to take the bottom cover off. Now supposedly this issue has been addressed by JJ and the new quads have normal sized pins but there still lurks the possibility of some of the older stock still being on some dealers shelves. Genalex Gold Lion now makes a KT77 tube but I have not heard this tube so in all honesty I cannot comment on its sound.
The ElectroHarmonix KT88's that you have in your amp are great sounding tubes for the money and a real good value.
Right now IMHO the best tubes for the ST-70 either a stock amp or a VTA boarded ST-70 amp like yours are the Genalex Gold Lion KT66's. They are a little on the expensive side - about $140 a quad. I use them in my own personal ST-70 and really like the sound. I bias them at .450 volts DC per tube. The Genalex KT77's are relatively new and are as expensive right now as the KT88's - about $200 a quad. I don't think the extra of $60 for the Gold Lion KT77's or KT88's is really worth it at least in an ST-70 based amp. In a Dynaco Mark III or my ST-120, which are 60 WPC amps and have 475 volts DC on the plates of the Mark III and 495 volts on the plates of the ST-120, you NEED the extra durability provided by a KT88 or 6550 tube. The KT88 and 6550 tubes are designed to "look at" 475+ volts on pin 3 (plate) and pin 4 (ultralinear screen tap). An ST-70 amp only has 410 - 430 volts on pins 3 and 4.
Bob
VTA boarded ST-70 with Genalex Gold Lion KT66 output tubes.
Bob Latino- Admin
- Posts : 3262
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Location : Massachusetts
Bob,
I know that you've retired, and might not see this, but I have to try. I do have a KT88s in my VTA70 with the stock PT. Could these tubes be run in short intervals and biased on the lower side as to not stress out the PT/OT?
Thanks,
Jon
You could try the KT88's in your ST-70 with a VTA driver board but they may not bias properly at .400 VDC per each output tube. If you can't get the bias down low enough remove the 7.5 K resistor in R39 and put in a lower value > maybe about 3K and the KT88's should bias properly.
Bob
I know that you've retired, and might not see this, but I have to try. I do have a KT88s in my VTA70 with the stock PT. Could these tubes be run in short intervals and biased on the lower side as to not stress out the PT/OT?
Thanks,
Jon
You could try the KT88's in your ST-70 with a VTA driver board but they may not bias properly at .400 VDC per each output tube. If you can't get the bias down low enough remove the 7.5 K resistor in R39 and put in a lower value > maybe about 3K and the KT88's should bias properly.
Bob
fedoragent- Posts : 57
Join date : 2013-09-14
Bob,
It worked like a champ, however instead of .400 it settled all at .410 even after 30 to 45 min. I'm also using a 5U4GB since they're like popcorn and I have a bunch of them. It sounds great. Thanks for the tip and hope you are doing well.
Thanks,
Jon
It worked like a champ, however instead of .400 it settled all at .410 even after 30 to 45 min. I'm also using a 5U4GB since they're like popcorn and I have a bunch of them. It sounds great. Thanks for the tip and hope you are doing well.
Thanks,
Jon
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fedoragent- Posts : 57
Join date : 2013-09-14
Bob Latino wrote:Bob,
I know that you've retired, and might not see this, but I have to try. I do have a KT88s in my VTA70 with the stock PT. Could these tubes be run in short intervals and biased on the lower side as to not stress out the PT/OT?
Thanks,
Jon
You could try the KT88's in your ST-70 with a VTA driver board but they may not bias properly at .400 VDC per each output tube. If you can't get the bias down low enough remove the 7.5 K resistor in R39 and put in a lower value > maybe about 3K and the KT88's should bias properly.
Bob
Bob,
I'll be danged, the KT88s worked out and biased at .400 on the dot. Worked like a charm. However, as I'm nearing towards 62 years of these OT and PTs (they're literally stamped 1961) I think it may be time for new PT and OT. I want to preserve the originals for a separate project for another time.
Happy Holidays Bob!
Cheers,
Jon
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