Tube Amps / Music Electronics
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|10/23/1998 9:19 AM|
||Hi-Watt Custom 100 bias question|
The Hi-Watt Custom 100 has a fixed bias voltage as standard. I'm going to need to modify it as the plates are glowing in my new Sovteks. The current is about 57mA per tube at the moment.
1. Are these amps supposed to be Class A? My one most definitely is.
2. Can anyone suggest a realistic bias setting that's not going to limit valve life? This isn't much like my Marshalls!
3. Which of the bias circuit resistors is the safest/best to change - and can I use a 0.6 Watt.
Doc, Ted, anyone else who has helped me out before - I'd really appreciate your thoughts.
|10/23/1998 9:44 AM|
1. Are these amps supposed to be Class A? My one most
>2. Can anyone suggest a realistic bias setting that's not going
to limit valve life? This isn't much like my Marshalls!
About 35 to 40ma per tube sounds good in my DR504, your mileage may vary with the big un.
3. Which of the bias circuit resistors is the safest/best to
change - and can I use a 0.6 Watt.
The series one is fine to change. I soldered a small ceremet into mine in parallel with the appropriate resistor to get the bias right. Works pretty good, but I sure wasn't too happy about soldering on one of these works of art.
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|10/23/1998 11:33 AM|
Actually, Hiwatts are intended to be biased to where a decent part of the operating curve falls in the class 'A' region. You really need to read this document by the Audio Brothers if you want some good info on Hiwatt biasing:
57ma is excessive. Off the top of my head, the B+ in Hiwatts I believe should ba around 460V, and the specified grid bias voltage is -37. If your amp's components and voltages check ok as per the schematic, you will want to make that mod to drop the bias to around 38-40ma, which gives about 70% of dissipation at idle, and sounds great in the 100W. There is a schematic for your amp, and even a description of a mod for altering the bias in Hiwatts in Pittman's "The Tube Amp Book", so please get that book if you don't already have it.
|10/23/1998 12:23 PM|
Hey Ted. I suppose you could argue that Fenders are biased near class A too, at least the way we do it these days... AB1 is fairly close to class A anyway isn't it? Don't remember, I should look it up...
For schematics of Hiwatts, I've got some up at my page at www.u.arizona.edu/~acruhl/schematics.htm. I don't have the power supplies up yet though, I'm working on that. The main thing to remember is the 100 watters have a seperate bias tap, where the 50 watters come off the HV winding.
|10/23/1998 2:19 PM|
I think most fixed bias Fender amps, from the 60's anyway, were designed to operate class AB1, but if the bias voltage is lowered sufficiently they are operating class AB2, which means there's grid current flowing in the output tube circuit. I think that's because the driver tube is supplying a signal voltage that exceeds the bias voltage. Guessing a bit here but looking in the RCA manual for the 6L6G specs, the only difference between class AB1 and AB2 is the amount of drive voltage to the grid, and I figure lowering the bias voltage is much the same as increasing the drive voltage, after all as the bias is lowered, the amount of grid signal to get to full power is lowered. So there's going to be some drive voltage in reserve which will put the tube into class AB2. AB2 is closer to A than AB1 is, although I've got a feeling that these amps aren't really in AB2, they're just trying to be, as the driver circuit was designed to operate into a higher impedance load than they're seeing. Perhaps someone can shed some light on this?...Jerry
|10/23/1998 2:51 PM|
||Re: Hi-Watt Custom 100 bias question (long)|
Oddly enough, I just finished overhauling one of these. I still have to mount everything back before the customer gets it. He complained that it ate tubes, and so he left it unused for 5 years. What a shame. These things are built like a tank. They are extremely hard to trace out, but it was worth it in this case, as the amp in question had been modified for higher gain, and the customer wanted it back to stock original sounding.
The problem with using these amps with newer tubes are threefold. First the power transformers in some produce much higher voltages on the secondarys in the 115V US setting. The one I have has 525V B+ unlike the 460V stated on the schematic. This isn't a problem with the plates, but it it means a higher voltage on the screen as well. Which leads us to problem number 2. The screen resistors are basically 570 Ohms (470 in series with 4 100's to each screen). At voltages as high as this amp, this does not produce a voltage drop that keeps the screen voltage low enough for reliable operation of EL34's. And finally the problem you've found the fixed bias is to low (voltage) to keep the idle current in check.
So the question is how to fix all this. Considering that the amp is hard to trace, and a work of art, any changes made should be done with caution, and be carefully planed out.
First the high voltage. You could safely drop the voltage using a number of methods. There are numerous posts here about reducing B+ so I won't repeat that here. I however left the voltage as is though, because it isn't really a problem with any of the components voltage ratings, except the screens which are derived from this.
Next the screens. I replaced the 100 Ohm resistors with 1K 5W cement Power Resistors and used a jumper where the 470 Ohm resistor was. This is not absolutely necessary, but it will make the amp more reliable when used consistently at loud volumes. Some people claim this changes the tone of an amp, but I can't here it, and I'm very picky. I suggest you try it both ways, and if your happy with the 1K's leave them in there.
Now for the Bias supply. I measured the voltage as 37V with no Power Tubes. This doesn't even come close to keeping the current in check, so I doubted that changing any resistors would help. I decided that a voltage doubler circuit would need to be used. Being as lazy as I am, I decided to check a few of the books I have, before designing a circuit from scratch. The Ultimate Tone II had just what I needed. On page 3-16 is a doubler circuit with bias balance, or individual tube bias. Unfortunately, I don't have a scanner so I can't give you the schematic, perhaps someone else can?... One thing not in the schematic though, is which wire is currently the ground leg of the bias winding. It will need to be moved from ground to middle of the two caps in the doubler. In the amp I have, the bias winding wires were both purple, and the ground leg is soldered to the chassis by one of the power tubes.
Anyway, the amp sounds great, and works fine now. Hope this helps.
|10/23/1998 4:10 PM|
When I come across an amp that needs seriously more voltage in the bias circuit, I just run it of the high voltage taps a la 50 watt Hiwatt or Princeton. It's just a few resistors that need to be changed. But I never did it to a Hiwatt...
525 volts with 1k 5watt screens shouldn't be a problem in that amp...
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