Power Tube Guru Needed

[Avogadro33]

Power Tube Guru Needed

Assume we have a Hi-Fi stereo amplifier designed for, say, the 7189 power tube. That tube has a plate voltage rating of 400V. Assume the schematic for the amplifier shows 420V going to the 7189 power tube. That seems a good and reasonable variance and all is well.

Question 1:

Assuming the amp has been turned on for a bit and is at operating temperature. Is 400V (or 420V) the plate voltage of the 7189 tube at all times, including when the amp output is at low volume, or is 400 (420V) only when the amp is putting out something near full power?

Question 2:

If this amp were played at low volume (something like the volume control at the 9-o’clock position), would the actual plate voltage of the power tubes still be in the 400V range, or something much lower?

Question 3:

Think about the 6BQ5 power tube, which we understand is in most ways like the 7189, but the 6BQ5 has plate voltage rating of 300V. Most agree that a 6BQ5 tube in this amp, designed for the 7189, would be expected to have a much-shortened life. But, depending on the answers to questions 1 and 2 above, assume further that this amp were used only as a secondary system: low volume in a bedroom or dining room. Would the amplifier still be feeding 420V to the 6BQ5 tubes, or due to the low-volume only might the plate voltage be much lower, and therefore the 6BQ5 perfectly acceptable?

Question 4:

Since this amp is, or is capable of, supplying 420V to the power tube, and it is designed for the 7189 power tube, might using a 6BQ5 tube cause stress to any other part of the amp? I’m thinking of stress on the transformers.

That’s it. I’d appreciate any knowledgeable assistance.

Howard

 

[maxhifi]

Power Tube Guru Needed

Assume we have a Hi-Fi stereo amplifier designed for, say, the 7189 power tube. That tube has a plate voltage rating of 400V. Assume the schematic for the amplifier shows 420V going to the 7189 power tube. That seems a good and reasonable variance and all is well.

Question 1:

Assuming the amp has been turned on for a bit and is at operating temperature. Is 400V (or 420V) the plate voltage of the 7189 tube at all times, including when the amp output is at low volume, or is 400 (420V) only when the amp is putting out something near full power?

At all times

Question 2:

If this amp were played at low volume (something like the volume control at the 9-o’clock position), would the actual plate voltage of the power tubes still be in the 400V range, or something much lower?

Still in the 400V range

Question 3:

Think about the 6BQ5 power tube, which we understand is in most ways like the 7189, but the 6BQ5 has plate voltage rating of 300V. Most agree that a 6BQ5 tube in this amp, designed for the 7189, would be expected to have a much-shortened life. But, depending on the answers to questions 1 and 2 above, assume further that this amp were used only as a secondary system: low volume in a bedroom or dining room. Would the amplifier still be feeding 420V to the 6BQ5 tubes, or due to the low-volume only might the plate voltage be much lower, and therefore the 6BQ5 perfectly acceptable?

Some 6BQ5 tubes are actually able to stand up to the higher voltage while others arent

Question 4:

Since this amp is, or is capable of, supplying 420V to the power tube, and it is designed for the 7189 power tube, might using a 6BQ5 tube cause stress to any other part of the amp? I’m thinking of stress on the transformers.

if the 6BQ5 fails due to the higher than rated voltage, it could damage the amplifier, yes.
 

That’s it. I’d appreciate any knowledgeable assistance.

Howard

Think about voltage like water pressure. If you have 100 psi of water pressure, the pipes and faucets have to withstand this pressure no matter if you're filling a swimming pool or brushing your teeth

 

[Avogadro33]

maxhifi:

Thank you. I now understand the matter much better.

I believe I'm going to refill the swimming pool, brush my teeth, then go shopping for a set of 7189s (or good 400V substitute), as the 6BQ5s currently in my just-acquired Scott 222C amp, though they've been in there for years, might be a ticking time bomb.

Thanks again,

Howard

 

[gadget73]

if anything the voltage drops at higher volume just because the output current increases and puts load on the power supply.

I wouldn't really be concerned about tubes just because they are old. If they are in good working shape there isn't a lot of reason to not run them. Spares are a smart idea though.

 

[spantou01]

Have you read the thread on Dave Gillespie’s Fisher SA-100 clone? You will find it good reading and it deals with your questions thoroughly.

This Fisher clone, like the classic Fishers before it used the EL84 family at elevated voltages without issues, although the EFB (tm) biasing scheme helps. Dave’s clone puts respectively 435 vdc and 330 Vdc across the plate and cathode and screen and cathode. Dave used regular EL84s as power tubes (although 7189s would not hurt, except the pocketbook for NOS) at these voltages without issues. However, he controled the dissipations carefully. The thread is here: https://www.audiokarma.org/forums/i...fisher-sa-100-clone-in-progress.466229/page-4

The discussion about EL84 and higher that specified plate voltage start at post #110.

 

[Tom Bavis]

hers before it used the EL84 family at elevated voltages without issues, although the EFB (tm) biasing scheme helps. Dave’s clone puts respectively 435 vdc and 330 Vdc across the plate and cathode and screen and cathode. Dave used regilar EL84s as power tubes (although 7189s would not hurt, except the pocketbook for NOS) at these voltages without issues. How

Actually, at full output the voltage at the plate is reaching about 750V momentarily on audio peaks (dropping to around 50V in between). With no audio, it's a steady 400V. The average voltage is 400 V in either case.

 

[Mr Boochie]

e

Assuming the amp has been turned on for a bit and is at operating temperature. Is 400V (or 420V) the plate voltage of the 7189 tube at all times, including when the amp output is at low volume, or is 400 (420V) only when the amp is putting out something near full power?

Howard

The Class of operation of the output stage will also affect the heat production. Whether you have a push-pull or single-ended amp can also be a factor. Here are some articles on various audio classes and topologies.

Classes.

Push-pull.

Single-ended.

 

[gadget73]

Actually, at full output the voltage at the plate is reaching about 750V momentarily on audio peaks (dropping to around 50V in between). With no audio, it's a steady 400V. The average voltage is 400 V in either case.

entirely true. You get the AC signal on top of the DC signal. I was just thinking of the DC voltage itself, if you measure that right at the power supply, you're likely to see it drop as power output increases. the instant voltage at the plate is quite different from the DC output of the supply though.