Tremolux bias mod

[Randall]

I have a tweed Tremolux coming in soon to play with different rectifiers to see if it changes anything to this customer's liking. His amp has 6BQ5's and a GZ34. He wants to try lowering the B+ with a 5AU4 or 5Y3. I'm thinking I'll need to adjust the bias supply voltage if we drop the B+ down that much. First of all is this true? And second, what is the best way to do this? I'm thinking the 56K in the bias supply. Can I lift it and clip in a 50KB pot to dial in the idle current?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[nosaj]

I have a tweed Tremolux coming in soon to play with different rectifiers to see if it changes anything to this customer's liking. His amp has 6BQ5's and a GZ34. He wants to try lowering the B+ with a 5AU4 or 5Y3. I'm thinking I'll need to adjust the bias supply voltage if we drop the B+ down that much. First of all is this true? And second, what is the best way to do this? I'm thinking the 56K in the bias supply. Can I lift it and clip in a 50KB pot to dial in the idle current?

https://www.thetubestore.com/lib/the...-Schematic.pdf

Here ya go another site but from Tubeswell here down in Aussie land.
https://el34world.com/Forum/index.php?topic=9243.0
nosaj

[Chuck H]

FWIW I think that even with 398V on the plates (and screens!) that -22V is pretty icy. Not that you can't run them like that, but regardless of what anyone else has to say about it EL84's just sound better running on the hot side. The Blues Jr. has only 330Vp, but only -10.7V of bias. Mesa Subway amps run about the same as the Tremolux and bias at -11V. My own experience designing with this tube is limited to cathode bias, but I'm running them at 365Vp and about 90% dissipation at idle.

Plus, consider that you'll be lowering the voltage. So you'll have to increase current a bit for a given dissipation at idle. I might use a 50k pot (wired like a rheostat as Tubeswell suggested) in series a 22k resistor. Something like 39k for the series resistor might not allow you to dial down the bias voltage if that ends up being desirable. And I think it probably will.

JM2C

[Randall]

The owner tells me the plate voltages are sitting at 484v, which is the primary concern. He says it sounds stiff, and wants to try to get closer to the 398v spec. He doesn't want to add a bias pot, I was just thinking I'd use one to find the value resistor to sub that closest matches it. He's quite knowledgeable, and does his homework, so this will be me just helping him to go where he wants to go but can't do it himself.

[nosaj]

The owner tells me the plate voltages are sitting at 484v, which is the primary concern. He says it sounds stiff, and wants to try to get closer to the 398v spec. He doesn't want to add a bias pot, I was just thinking I'd use one to fine the value resistor that closest matches it. He's quite knowledgeable, and does his homework, so this will be me just helping him to go where he wants to go but can't do it himself.

How are the power supply caps? Health wise and age wise
Schematic shows 450v caps
nosaj

[Chuck H]

If there's actually 484V on the plates then something is hinky. Maybe one power tube isn't conducting?

If he's knowledgeable why doesn't he want a bias pot in there?

Check the power transformer to see that it hasn't been replaced with a later transformer intended for different tubes. The Tremolux had several incarnations with different power tubes and PT's

If both tubes are conducting at anything resembling a normal idle current and the Vp is 484 a 5u4 tube isn't going to get you there.

This is a repair, straight up. Because there's something wrong with the amp.

[Enzo]

484v? I'd surely verify his reporting is accurate and properly done - ie with tube installed etc.

[Chuck H]

484v? I'd surely verify his reporting is accurate and properly done - ie with tube installed etc.

Ah! This ^^^^^^^^^^^^^^^^^^

[Randall]

All will be revealed once it comes to my bench. But the question really was about how to sort out the proper bias resistor, which I think was pointed out early here.

[nosaj]

All will be revealed once it comes to my bench. But the question really was about how to sort out the proper bias resistor, which I think was pointed out early here.

But what you have to remember is you are not the only viewer of your thread and for us to not point out inconsistancies in your provided information is doing everyone a disservice when the threads are viewed in a mentoring aspect of troubleshooting. But do please follow up on this particular amp.
nosaj

[Enzo]

I'm thinking I'll need to adjust the bias supply voltage if we drop the B+ down that much. First of all is this true?

No way to know until we know what the voltage is and your target dissipation. And we do not yet know if we need to do anything at all. If you do find 484v there, then we want to know why, but aside from that, we have no idea how hot that results in. It might turn out that if it is hot, dropping the B+ might put it exactly in line. On the other hand, we may not really have 484v and the existing B+ could be just what we want.


yes, changing the 56k resistor adjusts the B+. If you lift it and add a pot in series with it, then you can only raise the bias voltage - ie make it run cooler. If you replace the resistor with a 50k pot, then the coolest setting will be hotter than the stock. The thing to do is replace the 56k with something middling like a 33k and then add the 50k pot in series with that, so you can now adjust the 56k's place from 33k to 83k.

[Chuck H]

yes, changing the 56k resistor adjusts the B+. If you lift it and add a pot in series with it, then you can only raise the bias voltage - ie make it run cooler. If you replace the resistor with a 50k pot, then the coolest setting will be hotter than the stock. The thing to do is replace the 56k with something middling like a 33k and then add the 50k pot in series with that, so you can now adjust the 56k's place from 33k to 83k.

And because having something like -30V bias on EL84's is sort of unheard of I suggested a 22k resistor with a 50k pot. Which should allow adjustment down into the -teens. But if there IS 484V on the tubes then a 5U4 is only going to get us to about 450V. In which case it'll take something like -30V if the tubes are to survive at all!

[Enzo]

I'll believe your numbers. I just picked 33k as something near the middle of the 56k. The voltage division math is sismple enough, so the underlying point was to use a resistor and pot to create a useful range of bias. 10 to 30 is way more useful than 20 to zero.

[Chuck H]

10 to 30 is way more useful than 20 to zero.

Positively.

Er, ah,.. I mean negatively?

[g1]

If there's 484V there I'm going to want to know what the main caps are (voltage rating).