Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jojokeo on August 02, 2018, 01:14:16 pm
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Upon working recently on a HRD and looking at the schem there's an oddity that does not add up?
It states the -bias voltage to be -50v BUT on the output board's "official" biasing point at the 1ohm resistors it lists the biasing to be set to 60mV.
The "weirdness" issue is that the -50v setting is far to the left of the biasing pot and the 60mV setting is far to the right side! Obviously this does not make a whole lot sense? At least to me...
I biased the amp to 60mV (hotter) and called it good. The amp sounds very tight and punchy after adding additional capacitance to the filter caps along with re-biasing V1A so it runs cleaner on the clean channel, subbed an AT7 in pi slot and the owner is happy as can be.
But am wondering why the big discrepancy regarding the two different biasing points?
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Jojooooooooooooo
So, what is the range of negative voltage that you can get from the pot?...like -50V to -35V or something like that?
It sounds like you're saying that when the bias pot is turned away from -50V to it's opposite setting then you get the actual target bias reading of 60mV across 1 ohm. What is the voltage coming off the pot at that opposite setting?
If it really is around -35V(guessed number) then that's just where your specific set of tubes is biasing @ 60mA.
You dig?
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No discrepancy. -50V is the bias voltage applied to the grid. 6L6 is gonna want around -50V. 60mV represents 60mA cathode current.
Read note 5.
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I didn't read both voltage and current ranges throughout the bias pot. I hooked up my bias meter reading plate voltages and current readings directly with the tubes plugged into their sockets. To get -50V the setting was to the far left of the bias pot.
I then hooked up my DVM to the suggested test point per schematic, set for mV and to get to 60mV the bias pot had to be rotated to the far right / clockwise. Listening to the floor noise as I did so, I could hear it get louder as the bias pot turned more clockwise.
Steve, I just noticed something re-reading you suggestion but in note 4 that I didn't see before? I did everything except I didn't select the (yellow) drive mode channel? I left the amp in the clean channel mode. Could this be a very big deal or the difference??? :w2:
As far as "note 5" - are you saying that the bias pot reading therefore the -V reading "is for reference only?"
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Note 4 simply tells you how the amp was set to make all the voltage readings. If you set your amp to these same conditions you should get the same voltage readings.
Note 5 tells you everything you need to properly set the bias on this amp to Fender specs. You don't need a bias probe. TP30 is connected to the cathodes of the output tubes. There is also a 1Ω resistor between the cathodes and ground. So, when you adjust the bias pot for 60mV at TP30, you are done. 60mV represents 60mA current flowing through the tubes. Assuming the tubes are exactly matched, 30mA flows through one tube and 30mA flows through the other.
Look at the bias pot on the schematic and notice the wiper has a notation... C-, -50V, and note 5. When you turn the bias pot that -50V will change. This voltage is connected to pins 5 of the output tubes. This voltage controls the 60mV that you measure at TP30.When you get the desired 60mV at TP30, the -50V seen at the wiper of the bias pot will likely no longer be -50V, but it will be in the neighborhood. That's why note 5 says it's for reference only.
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Got it and basically what I was thinking too about the -50v documented on the schem when I used and biased to the recommended 60mV. But as originally stated the two documented values of bias referenced were in reality on opposite sides of the bias pot’s rotation. I thought to mention it to the forum and question it. Thank you as always Steve!
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So, when you set the bias pot to give 60mV at TP30, what is the voltage at the wiper of the bias pot? (May be easier to just measure the voltage at pin 5 of the output tubes)
Maybe you have some weak tubes that require setting the negative bias to a low value to get 60mV at TP30.
Also, be aware that when using a bias probe that you would set the bias to achieve 30mV(or 30mA) for each tube. This will be the same as setting for 60mV at TP30.
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So, when you set the bias pot to give 60mV at TP30, what is the voltage at the wiper of the bias pot? (May be easier to just measure the voltage at pin 5 of the output tubes)
Maybe you have some weak tubes that require setting the negative bias to a low value to get 60mV at TP30.
Also, be aware that when using a bias probe that you would set the bias to achieve 30mV(or 30mA) for each tube. This will be the same as setting for 60mV at TP30.
Roger that! Amp was serviced initially by another tech which couldn't seem to give customer what he wanted (more punch, headroom on channel 1, etc.) After replacing all tubes (new JJ 6L6s for what we're talking about) to give you an idea - I initially found an AY7 in V1, an AT7 in V2 (WTF?), and an AX7 for V3 (pi)...so not all techs know what they're doing and this one lost a good customer with lots of playing friends.
The amp was to be picked up already this week but it gives me another chance to check this stuff out again. I've got a Bias Rite type of meter not really a bias probe - maybe thought of as two different things? But I know what you mean. I used my DVM when reading the wiper voltage anyway. I will check this more thoroughly tomorrow (seeing Boston tribute band tonight, will be fun!)
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Almost forgot to update this:
I re-checked and tested the two bias settings:
Adjusting the negative voltage to -50Vdc at R59/R60 (most of the way left of the bias pot) - I got 27.9mV reading at suggested bias test point 30.
Adjusting bias test point 30 to 60mV (the recommended setting) was at most of the way to the right of the bias pot - then measured at R59/R60 I got -42.8Vdc
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Things, especially tubes vary, that's why there's bias pot :smiley:
If -50V bias was good for every tube and mains voltage etc, they could just stick zener (or otherwise) regulated bias supply and call it good.
But the -50V is just a typical / nominal reading.
What matters is the idle plate / cathode current,and in the tech documents they're thoughtful enough to tell us what the designer wanted it to be :worthy1:
It doesn't work out to be 70%, surely some mistake :w2: (joke!).