What is the correct bias voltage for KT 66 tubes in ST 70 with the VTA octal board modification? Are any circuit changes required? Currently running KT 77 tubes at 0.45V.
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peterh
texbychoice
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Correct Bias Voltage for KT 66 tubes
texbychoice- Posts : 21
Join date : 2020-12-03
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
Correct bias is the bias with minimum dist that is less then the maximum plate loss. Lackingtexbychoice wrote:What is the correct bias voltage for KT 66 tubes in ST 70 with the VTA octal board modification? Are any circuit changes required? Currently running KT 77 tubes at 0.45V.
measurment then use 0.5V as with EL34.
Note that grid resistors should be replaced to 100k in order for safe operation.
texbychoice- Posts : 21
Join date : 2020-12-03
peterh wrote:Correct bias is the bias with minimum dist that is less then the maximum plate loss. Lackingtexbychoice wrote:What is the correct bias voltage for KT 66 tubes in ST 70 with the VTA octal board modification? Are any circuit changes required? Currently running KT 77 tubes at 0.45V.
measurment then use 0.5V as with EL34.
Note that grid resistors should be replaced to 100k in order for safe operation.
To be sure I understand. You are saying replace the 1K resistor between pins 5 and 6 with a 100K resistor?
peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
texbychoice wrote:peterh wrote:Correct bias is the bias with minimum dist that is less then the maximum plate loss. Lackingtexbychoice wrote:What is the correct bias voltage for KT 66 tubes in ST 70 with the VTA octal board modification? Are any circuit changes required? Currently running KT 77 tubes at 0.45V.
measurment then use 0.5V as with EL34.
Note that grid resistors should be replaced to 100k in order for safe operation.
To be sure I understand. You are saying replace the 1K resistor between pins 5 and 6 with a 100K resistor?
NO, replace the 270k with 100k ( R29 and R31 on some schematic)
texbychoice- Posts : 21
Join date : 2020-12-03
peterh wrote:texbychoice wrote:peterh wrote:Correct bias is the bias with minimum dist that is less then the maximum plate loss. Lackingtexbychoice wrote:What is the correct bias voltage for KT 66 tubes in ST 70 with the VTA octal board modification? Are any circuit changes required? Currently running KT 77 tubes at 0.45V.
measurment then use 0.5V as with EL34.
Note that grid resistors should be replaced to 100k in order for safe operation.
To be sure I understand. You are saying replace the 1K resistor between pins 5 and 6 with a 100K resistor?
NO, replace the 270k with 100k ( R29 and R31 on some schematic)
Those are already 100K on my VTA Octal board. Thanks.
arremondo- Posts : 21
Join date : 2021-03-11
According to Bob Latino's post elsewhere on this site ( https://dynacotubeaudio.forumotion.com/t3043-vta-st-70-vta-st-120-and-vta-m-125-amplifier-information ), .45VDC for KT66's.
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knotscott- Posts : 62
Join date : 2019-12-28
Location : Rochester, NY
I bias my KT66 and 7581A at 0.45VDC on a Dyna/VTA 70 (per Bob Latino), and they sound wonderful.
Mazen0822- Posts : 11
Join date : 2022-08-20
knotscott wrote:I bias my KT66 and 7581A at 0.45VDC on a Dyna/VTA 70 (per Bob Latino), and they sound wonderful.
same, been running them like this for 2years now with zero issues. love the way they sound.
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Bob Latino- Admin
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Location : Massachusetts
For a VTA ST-70 .40 VDC is the proper bias point for EL34 output tubes. If you use KT66 or KT77 output tubes, you can raise the bias point to .45 VDC. Raising the bias point higher than the recommended bias points will shorten tube life.
A little history on the bias points of the ORIGINAL Dynaco ST-70 > Dynaco used a 15.6 ohm resistor from pin # 8 of each pair of EL34's to chassis ground on each channel. Dynaco recommended a bias point of 1.56 volts DC for each channel. This allowed 100 milliamps of current for the PAIR of tubes for that channel or 50 milliamps per each EL34 output tube. 50 milliamps of bias for each EL34 output tube THEN (1959 to about 1990) worked out OK because all the output tubes back then were made in the USA, Great Britain or Germany. The EL34's made then had pretty durable metal alloys inside the tube and they were tested well and almost no one got a "bad" tube with a new Dynaco ST-70 kit with Mullard EL34's. At some point they stopped making vacuum tubes in the USA, Great Britain and Germany and in modern times now virtually all vacuum tubes are made in China, Russia or the Slovak Republic. They now use cheaper and thinner metal alloys on the insides of the tubes and they don't test the new tubes as well. The result is that the newer tubes are more prone to fail earlier. Running the bias UP makes the problem even worse .. Keeping the bias of any EL34 today at 40 milliamps per each output tube won't stress them out as much and these newer EL34's will last longer.
Bob
A little history on the bias points of the ORIGINAL Dynaco ST-70 > Dynaco used a 15.6 ohm resistor from pin # 8 of each pair of EL34's to chassis ground on each channel. Dynaco recommended a bias point of 1.56 volts DC for each channel. This allowed 100 milliamps of current for the PAIR of tubes for that channel or 50 milliamps per each EL34 output tube. 50 milliamps of bias for each EL34 output tube THEN (1959 to about 1990) worked out OK because all the output tubes back then were made in the USA, Great Britain or Germany. The EL34's made then had pretty durable metal alloys inside the tube and they were tested well and almost no one got a "bad" tube with a new Dynaco ST-70 kit with Mullard EL34's. At some point they stopped making vacuum tubes in the USA, Great Britain and Germany and in modern times now virtually all vacuum tubes are made in China, Russia or the Slovak Republic. They now use cheaper and thinner metal alloys on the insides of the tubes and they don't test the new tubes as well. The result is that the newer tubes are more prone to fail earlier. Running the bias UP makes the problem even worse .. Keeping the bias of any EL34 today at 40 milliamps per each output tube won't stress them out as much and these newer EL34's will last longer.
Bob
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texbychoice- Posts : 21
Join date : 2020-12-03
Bob Latino wrote:For a VTA ST-70 .40 VDC is the proper bias point for EL34 output tubes. If you use KT66 or KT77 output tubes, you can raise the bias point to .45 VDC. Raising the bias point higher than the recommended bias points will shorten tube life.
A little history on the bias points of the ORIGINAL Dynaco ST-70 > Dynaco used a 15.6 ohm resistor from pin # 8 of each pair of EL34's to chassis ground on each channel. Dynaco recommended a bias point of 1.56 volts DC for each channel. This allowed 100 milliamps of current for the PAIR of tubes for that channel or 50 milliamps per each EL34 output tube. 50 milliamps of bias for each EL34 output tube THEN (1959 to about 1990) worked out OK because all the output tubes back then were made in the USA, Great Britain or Germany. The EL34's made then had pretty durable metal alloys inside the tube and they were tested well and almost no one got a "bad" tube with a new Dynaco ST-70 kit with Mullard EL34's. At some point they stopped making vacuum tubes in the USA, Great Britain and Germany and in modern times now virtually all vacuum tubes are made in China, Russia or the Slovak Republic. They now use cheaper and thinner metal alloys on the insides of the tubes and they don't test the new tubes as well. The result is that the newer tubes are more prone to fail earlier. Running the bias UP makes the problem even worse .. Keeping the bias of any EL34 today at 40 milliamps per each output tube won't stress them out as much and these newer EL34's will last longer.
Bob
Thanks for the history. I run out of adjustment range before reaching 0.45V. Lowest can reach is 0.48 - 0.46. Changed the NFB resistor to 4.3K recently. With that NFB value and KT77 tubes could set bias easily at 0.40V. Switching to KT66 tubes experienced running out of adjustment range. However, sound quality is better with KT66 tubes. Does NFB value impact bias? Do I need to increase the NFB value?
Bob Latino- Admin
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Join date : 2008-11-26
Location : Massachusetts
If you want to alter the bias point on a VTA amp, try changing out R39 (see photo) R39 on a VTA ST-70 is usually a 10,000 (10K) ohm resistor. In general, if you need to raise the bias, lower the value of R39. If you need to lower the bias, put in a larger value resistor than the 10000 ohm resistor which is the normal value for a VTA ST-70. See photo below.
Bob
Bob
Last edited by Bob Latino on Sat Sep 28, 2024 8:50 am; edited 1 time in total
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peterh- Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden
Bob, the title should be "if you want to alter feedback ... " ?
texbychoice- Posts : 21
Join date : 2020-12-03
Bob Latino wrote:If you want to alter the bias point on a VTA amp, try changing out R39 (see photo) R39 on a VTA ST-70 is usually a 100000 (10K) ohm resistor. In general, if you need to raise the bias, lower the value of R39. If you need to lower the bias, put in a larger value resistor than the 10000 ohm resistor which is the normal value for a VTA ST-70. See photo below.
Bob
My VTA Octal Board kit came with 6.8K resistor for R39. Sounds like increase to 7.5K is the right direction so can set each tube bias at 0.45V? By the way, I installed 10 turn pots some time ago to allow finer bias adjustment.
texbychoice- Posts : 21
Join date : 2020-12-03
Changed R39 from 6.8K to 7.5K. Minimum bias voltage could achieve was 0.55V. Changed to 5.6K for R39. Then able to adjust as low as 0.30 V. Set at 0.45V.
Bob Latino- Admin
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peterh wrote:Bob, the title should be "if you want to alter feedback ... " ?
No ... R39 is the main bias resistor and should have a value of about 10K on a VTA ST-70. R7 is the feedback resistor and should have a value of 7.5K. R7 is the resistor to alter the feedback. You can alter this resistor if you want to experiment with the amount of feedback the circuit has. IMHO, if you substitute any resistor that is larger than about 10K the bass can get a little sloppy. If you substitute anything lower than about 3K, then the amp will sound a little less "tubelike". Altering the feedback resistor will also alter slightly the gain of the amp. Using a 10K resistor in place of the 7.5K resistor will make the amp sound very slightly louder. Dropping the value of R7 will drop the volume of the amp very slightly.
Below is a schematic of one channel of a VTA ST-70 without most of the resistor values included since this circuit is proprietary to Tubes4hifi.com. Note > You do get a full schematic with all resistor values if you purchase the VTA ST-70 kit.
Bob
texbychoice- Posts : 21
Join date : 2020-12-03
Bob Latino wrote:peterh wrote:Bob, the title should be "if you want to alter feedback ... " ?
No ... R39 is the main bias resistor and should have a value of about 10K on a VTA ST-70. R7 is the feedback resistor and should have a value of 7.5K. R7 is the resistor to alter the feedback. You can alter this resistor if you want to experiment with the amount of feedback the circuit has. IMHO, if you substitute any resistor that is larger than about 10K the bass can get a little sloppy. If you substitute anything lower than about 3K, then the amp will sound a little less "tubelike". Altering the feedback resistor will also alter slightly the gain of the amp. Using a 10K resistor in place of the 7.5K resistor will make the amp sound very slightly louder. Dropping the value of R7 will drop the volume of the amp very slightly.
Below is a schematic of one channel of a VTA ST-70 without most of the resistor values included since this circuit is proprietary to Tubes4hifi.com. Note > You do get a full schematic with all resistor values if you purchase the VTA ST-70 kit.
Bob
I understand the standard VTA and VTA Octal board are different. My Octal board kit came with 6.8K R39. Also, 6.8K R7 and R8. Why are those values different than the standard VTA board?