I have an HH VS Musician head that gives live a little bit of weird decay and fizz when using the gain especially with chords. The amp plays fine clean, but has problems with gain even if I just play clean and use a distortion pedal. I was told from somebody that the biasing was probably set a bit too cold and I'm getting crossover distortion. He said I need to add a pot or change resistor values and bias the amp correctly. This amp has a fixed bias, so there is no pot to adjust.
My problem is I've never done any biasing and I'm not sure where or how to measure the emitter resisters so I can figure out the voltage drop. There's 5 transistors on the power amp board which is the AS00014 board. Do I measure all the transistors at their closest resistor and look for voltage drops?
Any help is appreciated. Any links to any explanation for a circuit similar to this would be helpful. I'm a little bit rusty on my NPN and PNP transistors, and have found some help online but my circuit seems so different from all the examples. It's been awhile since I've done my electronics school equations to figure this stuff out. Unfortunately, I don't have an oscilloscope just a DMM.
I've replaced all the electrolytic caps and old opamps so I dont think it's any of those that are causing my problems. Here's the schematic. Thanks!
My problem is I've never done any biasing and I'm not sure where or how to measure the emitter resisters so I can figure out the voltage drop. There's 5 transistors on the power amp board which is the AS00014 board. Do I measure all the transistors at their closest resistor and look for voltage drops?
Any help is appreciated. Any links to any explanation for a circuit similar to this would be helpful. I'm a little bit rusty on my NPN and PNP transistors, and have found some help online but my circuit seems so different from all the examples. It's been awhile since I've done my electronics school equations to figure this stuff out. Unfortunately, I don't have an oscilloscope just a DMM.
I've replaced all the electrolytic caps and old opamps so I dont think it's any of those that are causing my problems. Here's the schematic. Thanks!
Attachments
Also, I realize you have to put the two pages above side by side for them to make any sense. Unfortunately, HH does not exist anymore so I can't get help with any service manuals about bias.
If it's playing fine clean, i doubt it is biasing. You would most certainly hear that on a clean signal.
You changed the opamps I see. My guess is one or more of tne new ones are oscillating. The amp needs to go on a scope to see what's going on. Tweaking the bias without knowing what else is happening could result in destruction.
Get it to a tech. This is not diy.
You changed the opamps I see. My guess is one or more of tne new ones are oscillating. The amp needs to go on a scope to see what's going on. Tweaking the bias without knowing what else is happening could result in destruction.
Get it to a tech. This is not diy.
It had this problem BEFORE I changed the opamps and caps. The sound has improved and doesn't sound as grainy, but I still have a weird decay. The person that recommended the bias is an expert on these amps from the UK.
Attached.
Bias is checked by first checking for NO-signal, no load, and near-Zero (<20mV) volts DC at the output.
Then use a battery-power meter across each 0.33r resistor R26 R27. Anything 20mV to 100mV is acceptable. Under 20mV may well be too-cold bias. Over 100mV would idle hot. (And you do not want to go anywhere near "hot" in this old design.)
Bias is set by the factory selecting a resistor R15. Larger is hotter. Assuming R15 is infinite (not fitted) we have around 1.8V bias, or 0.44V for each transistor of the 4-high output stage. It is likely that big devices Q1 Q2 are "off" at idle, Q6 Q7 pull the load through R25 R28 33 Ohms. Then you may only find 3mV-5mV across each 0.33r resistor, "cold".
i.e. this is NOT a high-quality High Fidelity amplifier. It may never be dead-clean on long note decay. And this may be the way it is without risky re-design.
BTW: this is a wonkly NFB system. There is negligible voltage feedback from the speaker, it acts as a hi-Z source, low damping. R6 R3 set IC1 gain. R31 senses speaker return current into R32 and Q1. H-Z is a very reasonable choice for a guitar amp, but is a brain-pain for circuit geeks accustomed to low-Z designs; also the split NFB loops.
Wow thank you so much! This is exactly what I needed. I checked R15 and R14 (since it is parallel) while in the circuit with no power on, and they read 78.3 K ohms. The schematic says R14 is 100K. I realized I probably should measure them out of the circuit. But you are right, this amp might just sound like this. I need to ask other users.
> they read 78.3 K ohms. The schematic says R14 is 100K
No. By squint and by doping-out what is happening, R14 is 100 Ohms, not 100K.
And your 78K reading worries me. Are you sure of the "K"?? Some meters do not make this obvious.
If the amplifier really had Ks of resistance there, it would already be toast. That is a REAL problem "biasing" big transistor amps: a mistake can do a LOT of damage very fast.
No. By squint and by doping-out what is happening, R14 is 100 Ohms, not 100K.
And your 78K reading worries me. Are you sure of the "K"?? Some meters do not make this obvious.
If the amplifier really had Ks of resistance there, it would already be toast. That is a REAL problem "biasing" big transistor amps: a mistake can do a LOT of damage very fast.
Sorry, you are right, its not K ohms. I was going by the schematic, that little "R" looks like a "K" 🙂 . I measured it with my DMM on the right setting and got 78.3. Wheres a safe place to put the positive and negative probes to measure R26 and R27? On this pcb they have "+" "-" "G" pins and then some with "C" "E" "B". I think that means Collector, emitter, and base for the transistors? But then there is a "S" pin. These are in the middle of the schematic because you put both parts together.
Well I checked R26 and R27 after leaving the amp on for over 10 minutes and I got around 16.8 mV for R26 and 16.5 mV for 27. According to your calculations this means the bias is a bit cold! This is what I suspected. Whats the best way that you recommend to set the bias? Should I change R15 to a different value or insert a pot somewhere? I've heard of people doing this by ear.
Replace R15 with 420 ohms. Be SURE your joints are good! This across the 100 Ohm R14 should now read 80.7 Ohms. Check power stage bias.
You may get to 470 Ohms at R15 before it runs warm. I would not go further.
You may get to 470 Ohms at R15 before it runs warm. I would not go further.
Well I was about to look into what you suggested...
The only thing I noticed is R14 on the pcb is different from the schematic! Its actually an 82 ohm resistor (not 100 ohms) and R15 is a 1K ohm resistor. Because they are parallel this equals the total resistance of 75.7 ohms which is close to what I got. I measured 78 ohms on R14 and R15. What should R15 be instead of 470 ohms since R14 is different?
The only thing I noticed is R14 on the pcb is different from the schematic! Its actually an 82 ohm resistor (not 100 ohms) and R15 is a 1K ohm resistor. Because they are parallel this equals the total resistance of 75.7 ohms which is close to what I got. I measured 78 ohms on R14 and R15. What should R15 be instead of 470 ohms since R14 is different?
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Thanks for your help. I tried what you suggested, removing R15 and unfortunately the sound didn't change that much at all. I still have a fizzy decay when I play chords. I measured R26 and R27 and they both were around 33mV. I'm starting to think that this amp just sounds this way, and it's the design. The valvesound feature has been updated by some other people, I might look into the mods they do. It's a shame they all are in the UK. I'm in this US. Thanks for trying, I've learned a lot about BJTs playing with this old amp.
Sorry late in the thread.
If still relevant, a few details:
1) do not mess with bias string, it´s a cvery old school design and there is more to it than meets the eye.
2) 17mA across emitter resistors is *fine*, above the crossover region, so that´s not your problem, look elsewhere
3) R15 which as you see has no written "numerical" value is most probably a Thermistor, its value lowers when heating.
It´s the old schoola´s approach to thermal compensation and it can not be replaced with *any* fixed resistor.
* Too low and amp will buzzy crossover distort .... which in some old designs (think VOX) was considered "acceptable" , go figure, because the alternative is:
* too high: amp sounds beautiful at low power, you go play LOUD and less than an hour later amp overheats and burns, because bias did not track heat sink temperature.
While a Thermistor lowers own resistance when heating, and lowers bias voltage.
So replace it where it belongs and leave amp original.
IF you have fizzy sound with drive/distortion, it most probably comes from the "secret sauce" AS00015/A module, the "tube distortion simulator", which as you see has NO circuit shown, it was a secret potted module, which among other goodies includes inductive equalizers and a FET "connected as a Tube".
Not bad for 1969/70 ... but we have come a long way since 🙂
And the Power Amp itself was a highly regarded Pro/Hi Fi very advanced one (again, for 1970) , so much so that it was used by the BBC as control room monitoring amplifier.
This is the "rogue" version, modded for Guitar, which includes mixed impedance feedback to increase output impedance and kill damping.
FWIW here´s one famous HH user: Marc Bolan (and his band T Rex):
YouTube
and your head (or a very similar model) is clearly seen, together with an early Orange cabinet.
If still relevant, a few details:
1) do not mess with bias string, it´s a cvery old school design and there is more to it than meets the eye.
2) 17mA across emitter resistors is *fine*, above the crossover region, so that´s not your problem, look elsewhere
3) R15 which as you see has no written "numerical" value is most probably a Thermistor, its value lowers when heating.
It´s the old schoola´s approach to thermal compensation and it can not be replaced with *any* fixed resistor.
* Too low and amp will buzzy crossover distort .... which in some old designs (think VOX) was considered "acceptable" , go figure, because the alternative is:
* too high: amp sounds beautiful at low power, you go play LOUD and less than an hour later amp overheats and burns, because bias did not track heat sink temperature.
While a Thermistor lowers own resistance when heating, and lowers bias voltage.
So replace it where it belongs and leave amp original.
IF you have fizzy sound with drive/distortion, it most probably comes from the "secret sauce" AS00015/A module, the "tube distortion simulator", which as you see has NO circuit shown, it was a secret potted module, which among other goodies includes inductive equalizers and a FET "connected as a Tube".
Not bad for 1969/70 ... but we have come a long way since 🙂
And the Power Amp itself was a highly regarded Pro/Hi Fi very advanced one (again, for 1970) , so much so that it was used by the BBC as control room monitoring amplifier.
This is the "rogue" version, modded for Guitar, which includes mixed impedance feedback to increase output impedance and kill damping.
FWIW here´s one famous HH user: Marc Bolan (and his band T Rex):
YouTube
and your head (or a very similar model) is clearly seen, together with an early Orange cabinet.
Thanks for your reply, and all the knowledge all about this device. After thinking about it more, I think I have two separate issues. High end fizz from both channels for different reasons.
One reason the amp doesn't like distortion boxes on the clean channel is I might be overloading it, probably worse when I use a humbucking guitar. This is why I get some high end fizz on that channel.
The other channel with the valvesound probably has high end fizz from the valvesound tone correction module. Probably not a big deal back in the 70s when this came out. I tried this head with a different cabinet, so I know it's not the speakers. Thanks for letting me know about R15. I'll probably put it back in. Also, at band volume these issues are less noticeable so I might just live with it.
Oh yes, I have seen that T Rex vid and it rocks! Makes me want to get a treble booster and plug it in.
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