I just picked up an ST70 with the 12AU7 VTA driver board. What bias voltage am I looking for on the EL34s?
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dmtparker
audiobill
stewdan
Ed Rosenberger
GP49
MexicoMike
Bob Latino
tygr1
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Bias voltage for ST70 with VTA driver board?
tygr1- Posts : 35
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Bob Latino- Admin
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Set each EL34 output tube's bias at .400 volts DC ..
Bob
Bob
tygr1- Posts : 35
Join date : 2014-12-08
Thank you Bob!
MexicoMike- Posts : 28
Join date : 2014-11-21
Just curious - I don't understand circuitry so forgive my ignorance...
On the stock 70, the bias for two EL34s is 1.56V so I would ASSUME (incorrectly?) that the bias for one tube would be 1/2 of that. So why would the individual bias be .4 instead of a bit under .8?
On the stock 70, the bias for two EL34s is 1.56V so I would ASSUME (incorrectly?) that the bias for one tube would be 1/2 of that. So why would the individual bias be .4 instead of a bit under .8?
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
Because there is a different amount of cathode current being specified, 40mA per tube vs. 80mA per tube. Dynaco's original called for more cathode current than what's being recommended by Bob and others. Lower cathode current increases tube life. It also heats and stresses the tube less. It is known that old stock, original Mullard EL34s were more rugged and durable than current production, including the imitation "Mullard" EL34s being made now, so the slightly higher distortion (I think you'd notice it only on test equipment, not by ear) from running at 40mA is probably a good tradeoff for improved tube reliability and service life.
MexicoMike- Posts : 28
Join date : 2014-11-21
Thanks for the explanation. Any idea what the "expected" EL34 tube life difference might be between the lower bias and original bias? I was under the impression that the EL34 wasn't really stressed that much in an original ST70. At least, I read that on the internet, so I assumed it MUST be true! 
GP49- Posts : 792
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I have no hard information about how much longer a tube would last at lower cathode current. The ratings for the original Mullard EL34 include a VERY high B+ of 800V; I'd expect that if you put 800V on a current-production EL34 from anybody, the chances are it would arc over. However, assuming that the B+ voltage is within reason, it will be heat from the idling cathode current that makes the difference. 40mA is HALF of 80mA and heat would be proportional to the amount of current, all else being equal. Power, therefore heat, is proportional to current.
This only affects the tubes at IDLE; when playing music the current is changing all the time, and mostly above the idling cathode current.
This only affects the tubes at IDLE; when playing music the current is changing all the time, and mostly above the idling cathode current.
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GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
I must admit, Peter is right. I have to stop trying to do calculations in my head using parts values from memory!!!
Forget that 80mA stuff...
Forget that 80mA stuff...
Ed Rosenberger- Posts : 16
Join date : 2014-10-23
Location : Gilboa, Ohio
- Post n°11
Re: Bias voltage for ST70 with VTA driver board?
by Ed Rosenberger Fri Dec 12, 2014 10:51 pm
Bob Latino wrote:Set each EL34 output tube's bias at .400 volts DC ..
Bob
This may be a dumb question but all output tube references to bias voltages (.4 vdc) that I've seen have been for EL34 tubes.
I'm using the VTA octal driver board and the instructions also call for a .4 vdc output bias when using EL34 tubes.
I have new EH 6CA7 output tubes.....so does the bias voltage remain the same for these four tubes?
I am not to the power-on stage yet.....just kinda looking ahead.
stewdan- Posts : 234
Join date : 2010-03-07
Age : 86
Location : Houston Texas
Hi Ed --- The 6CA7 is "equivalent" to the EL34, so the bias is the same.
Stew
Stew
Ed Rosenberger- Posts : 16
Join date : 2014-10-23
Location : Gilboa, Ohio
- Post n°13
Re: Bias voltage for ST70 with VTA driver board?
by Ed Rosenberger Sat Dec 13, 2014 2:09 am
stewdan wrote:Hi Ed --- The 6CA7 is "equivalent" to the EL34, so the bias is the same.
Stew
Okay Stew....thank you for taking the time to relieve one of my "don't have a clue" anxieties.
Wish knowledge could be gained through brain-wave osmosis.....I'm almost too old for using books
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audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
Books? What are those?
Ed Rosenberger- Posts : 16
Join date : 2014-10-23
Location : Gilboa, Ohio
- Post n°15
Re: Bias voltage for ST70 with VTA driver board?
by Ed Rosenberger Sat Dec 13, 2014 5:41 pm
audiobill wrote:Books? What are those?
Yeah....kinda gives away our extended age huh
. Actually I haven't been to a library or opened the cover of a book since I could type my topic into a search engine like google. Rather sad statement.If I end up wishing for better sound and want to try different output tubes (for some reason this is called "tube rolling"), I will need to find out how to tell if a tube is ("equivalent" to the EL34), before I can do anything productive.
I'll learn.....don't no how long I'll retain it though....
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dmtparker- Posts : 14
Join date : 2014-09-25
Location : Bocas del Toro, Panama
I have seen several places, your dictum to set bias @ 40mA/tube, but David Gillespie in his article on "Dynaco ST-70, Baseline Testing" says the following:Bob Latino wrote:Set each EL34 output tube's bias at .400 volts DC ..
Bob
"For example, with a Biaset voltage of 1.40 vdc (45 ma total current flow per tube), 1 kHz THD climbs nearly 55%. If you reduce it further to 1.25 vdc (40 ma per tube) it climbs another 170%, for a total of 225%! So, reducing the current draw of each tube by 6% of its rated cathode current, when it is already operating at just 30% of rated cathode current and 62% of rated plate dissipation, all to gain what, 225% more distortion? That is a very poor compromise to make, versus the very well thought out operating conditions that Hafler set up for the tubes to operate at."
That seems like an awful trade-off. What gives? Do you have different measurements of THD @ 40mA vs. 50mA? I'm sure my original ST-70 had 100's & 100's of hours on it @ 50mA bias as it was my main amp all through college and I never replaced the tubes.
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dmtparker wrote:I have seen several places, your dictum to set bias @ 40mA/tube, but David Gillespie in his article on "Dynaco ST-70, Baseline Testing" says the following:Bob Latino wrote:Set each EL34 output tube's bias at .400 volts DC ..
Bob
"For example, with a Biaset voltage of 1.40 vdc (45 ma total current flow per tube), 1 kHz THD climbs nearly 55%. If you reduce it further to 1.25 vdc (40 ma per tube) it climbs another 170%, for a total of 225%! So, reducing the current draw of each tube by 6% of its rated cathode current, when it is already operating at just 30% of rated cathode current and 62% of rated plate dissipation, all to gain what, 225% more distortion? That is a very poor compromise to make, versus the very well thought out operating conditions that Hafler set up for the tubes to operate at."
That seems like an awful trade-off. What gives? Do you have different measurements of THD @ 40mA vs. 50mA? I'm sure my original ST-70 had 100's & 100's of hours on it @ 50mA bias as it was my main amp all through college and I never replaced the tubes.
Yes - BUT - Mr. Gillespie is working with an ST-70 with a STOCK 7199 (or 6GH8A) driver board. All his facts and figures relate to the stock driver circuit. The VTA driver board is a different animal and has lower distorion than the stock driver board at all drive levels .. maybe biasing that 7199 driver circuit at higher bias levels (55 milliamps per each EL34) does reduce distortion but it will surely lead to shorter tube life than biasing at 40 milliamps.
Answer this one > If the original 7199 driver circuit was as "good" as Mr. Gillespie says, how come no other manufacturer besides Dynaco ever used this 7199 "one driver tube per channel" circuit ? Dynaco used this 7199 driver circuit not because it was good, but because it saved one tube per amp. A saved 7199 driver tube @ $1.50 (1960's pricing ) X 350,000 amps = over 1/2 million (1960's dollars) saved. When the whole ST-70 kit with tube cage and all tubes in 1960 costs $99 and you save $1.50 it means something.
Bob
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dmtparker- Posts : 14
Join date : 2014-09-25
Location : Bocas del Toro, Panama
Bob
I am confused. When setting the bias - either on the original or on a VTA mod - I thought you were only setting bias for the EL34's, NOT changing the bias on the driver board as well. If that is the case, Gillespie's eval is pertinent regardless of driver board. I am sure the VTA has much lower distortion than the 7199, but if you only change the bias to the EL34's, any CHANGE in distortion is coming from the EL34's as a result of different bias, NO?
Yes - BUT - Mr. Gillespie is working with an ST-70 with a STOCK 7199 (or 6GH8A) driver board. All his facts and figures relate to the stock driver circuit. The VTA driver board is a different animal and has lower distorion than the stock driver board at all drive levels .. maybe biasing that 7199 driver circuit at higher bias levels (55 milliamps per each EL34) does reduce distortion but it will surely lead to shorter tube life than biasing at 40 milliamps.
Bob
I am confused. When setting the bias - either on the original or on a VTA mod - I thought you were only setting bias for the EL34's, NOT changing the bias on the driver board as well. If that is the case, Gillespie's eval is pertinent regardless of driver board. I am sure the VTA has much lower distortion than the 7199, but if you only change the bias to the EL34's, any CHANGE in distortion is coming from the EL34's as a result of different bias, NO?
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What Mr. Gillespie is saying is that the stock driver circuit "requires" that the output tubes be at a higher bias setting in order for the stock driver circuit to drive the output tubes at a lower distortion level. The VTA driver circuit does not require that the output tubes be biased that high in order to get the most out of the driver circuit.
One of the "advantages" of biasing higher is that the amp will stay in class A (both output tubes conducting) at slightly higher volume levels before switching to class AB1 (each output tube of the pair conducting on only one half the cycle). When the amp is working in class A it WILL have somewhat lower distortion levels. This is really the only advantage of biasing at higher bias levels. The downside of higher bias levels is, of course, much shorter tube life. Dynaco knew that they had those extremely well made Mullard EL34's as their output tubes. These tubes handled the higher bias levels in stride. Today's output tubes are not as well made and do not handle higher higher biasing strategies as well as the original Mullard EL34's.
Bob
One of the "advantages" of biasing higher is that the amp will stay in class A (both output tubes conducting) at slightly higher volume levels before switching to class AB1 (each output tube of the pair conducting on only one half the cycle). When the amp is working in class A it WILL have somewhat lower distortion levels. This is really the only advantage of biasing at higher bias levels. The downside of higher bias levels is, of course, much shorter tube life. Dynaco knew that they had those extremely well made Mullard EL34's as their output tubes. These tubes handled the higher bias levels in stride. Today's output tubes are not as well made and do not handle higher higher biasing strategies as well as the original Mullard EL34's.
Bob
dmtparker- Posts : 14
Join date : 2014-09-25
Location : Bocas del Toro, Panama
Bob Latino wrote:What Mr. Gillespie is saying is that the stock driver circuit "requires" that the output tubes be at a higher bias setting in order for the stock driver circuit to drive the output tubes at a lower distortion level. The VTA driver circuit does not require that the output tubes be biased that high in order to get the most out of the driver circuit.
One of the "advantages" of biasing higher is that the amp will stay in class A (both output tubes conducting) at slightly higher volume levels before switching to class AB1 (each output tube of the pair conducting on only one half the cycle). When the amp is working in class A it WILL have somewhat lower distortion levels. This is really the only advantage of biasing at higher bias levels. The downside of higher bias levels is, of course, much shorter tube life. Dynaco knew that they had those extremely well made Mullard EL34's as their output tubes. These tubes handled the higher bias levels in stride. Today's output tubes are not as well made and do not handle higher higher biasing strategies as well as the original Mullard EL34's.
Bob
Thank-you. That makes sense to me. It was common for Dynaco to make the most of "special" parts in both their tube and (especially) ss amps - a typical Hafler 'most bang for your buck' approach. That, unfortunately, creates problems for us today not having access to such 'special' parts.
GP49- Posts : 792
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Bob Latino wrote:
Answer this one > If the original 7199 driver circuit was as "good" as Mr. Gillespie says, how come no other manufacturer besides Dynaco ever used this 7199 "one driver tube per channel" circuit?
Actually Dynaco was by no means the only manufacturer that used it.
H.H. Scott did, also (for example, using the 6GH8 in integrated amplifiers, driving 7591 and EL84 output tubes); and that's just off the top
of my head. I've no doubt that if more research were done, I could come up with others, too.
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
What is "the right" value of bias for a vta-70 with el34 ?
So far we have seen no figures that supports either value of bias. The fact that a lower bias would
enhance tube lifetime has no objection, but the effect on distortion is yet to be examined.
The effects are in two areas ( maybe more ) :
1/ crossover distortion, here the driver circuit has no influence since it is the powertubes that creates it. This
should be measured at low level ( 1w ?) and possibly with feedback disconnected.
2/ non-linear behaviour due to working point not symmetrical. This is a function of the whole amp
and also depending on output level , the higher output the more distortion. This is what D.Gillespie did
in the mentioned article.
What could be done here ? One could ask for Mr Latino to give Mr Gillespie to opportunity to rerun the tests with
a VTA-70 and possibly figure out what level of bias works best. It could be as easy as sending him a driver board.
Another possibility is that someone who has an amp and the proper instruments to do , preferably (1) above
as this is the most likely "real life" listening level.
I am in a possession of a hp3580A but i have no clean enough source to be able to have any meaningful figures.
Anyone takes this glove ?
So far we have seen no figures that supports either value of bias. The fact that a lower bias would
enhance tube lifetime has no objection, but the effect on distortion is yet to be examined.
The effects are in two areas ( maybe more ) :
1/ crossover distortion, here the driver circuit has no influence since it is the powertubes that creates it. This
should be measured at low level ( 1w ?) and possibly with feedback disconnected.
2/ non-linear behaviour due to working point not symmetrical. This is a function of the whole amp
and also depending on output level , the higher output the more distortion. This is what D.Gillespie did
in the mentioned article.
What could be done here ? One could ask for Mr Latino to give Mr Gillespie to opportunity to rerun the tests with
a VTA-70 and possibly figure out what level of bias works best. It could be as easy as sending him a driver board.
Another possibility is that someone who has an amp and the proper instruments to do , preferably (1) above
as this is the most likely "real life" listening level.
I am in a possession of a hp3580A but i have no clean enough source to be able to have any meaningful figures.
Anyone takes this glove ?
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Last edited by PeterCapo on Thu Dec 03, 2020 10:23 pm; edited 1 time in total
zx- Posts : 205
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I Well today I have only 3ea pr of Dynaco MK3s stock 6AN8s....an had 4-5 ST70..hhscott...Heath..all with 7199s .....for my old ears one tube has less,or more noise....hehe
Most of the newer input board...I have heard.. change the stock sound to much.....but that's just me...
But if we don't know if we don't go....right....have fun with tubes...life is short........
Thanks for the site Bob..................
Most of the newer input board...I have heard.. change the stock sound to much.....but that's just me...
But if we don't know if we don't go....right....have fun with tubes...life is short........
Thanks for the site Bob..................