Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yosser on April 04, 2026, 12:03:07 pm
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I have restored a Carlsboro 60 Par amplifier which I think sounds great as a guitar amplifier, however I have read that this amplifier has a reputation for not being pedal friendly, what exactly does that mean?
I have tried it with an ocd overdrive and it sounded as fine, but could the situation be improved?
I have attached a schematic, as you can see the input stage is different to a guitar amplifier with regard to the cathode ground and 3m3 resistors, I have read that they can be removed and a conventional fender style cathode biased arrangement can be installed?
Also the four volume controls seem redundant really as the master can be used for that.
The amplifier has a open airy ringing chime about it which seems to enhance the sustain of my stratocaster so wouldn't want to detract from that.
Should I attempt to modify it?
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Pedal friendly simply means the amp sounds good with pedals plugged into the input. If this were mine I would convert to cathode bias. Easy to do, at least on paper.
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Pedal friendly simply means the amp sounds good with pedals plugged into the input. If this were mine I would convert to cathode bias. Easy to do, at least on paper.
Thank you for posting the modification schematic, I think I might attempt to implement that
I have a couple of questions though, what is the purpose of the 3m3 resistors, is it to prevent rf getting through or parasitic oscillation and
are the 0.047uf caps there to prevent the input pots from being noisy when turned (dc)?
Will any of that cause me a problem if I convert it?
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... what is the purpose of the 3m3 resistors, is it to prevent rf getting through or parasitic oscillation
No, you may be thinking of 'grid stoppers'? The 100k input mixer resistors kinda act as that. The 3M3 are grid leak resistors, in this case they convert DC grid current into grid bias DC voltage.
In cathode bias stages, the grid leak is usually 1M.
are the 0.047uf caps there to prevent the input pots from being noisy when turned (dc)?
Those caps prevent variations in the DC resistance at the input affecting the grid bias DC voltage.
It's fine to tinker with your amp, but as you seem to really like it as it is, I suggest not to make big changes that will be a lot of work to reverse, without temporarily trying them out first.
Or make the stage for inputs 1 and 2 cathode bias, and leave the stage for inputs 3 and 4 alone, at least for the moment.
Just because something 'should' be better doesn't mean you'll like it more.
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Pedal friendly simply means the amp sounds good with pedals plugged into the input. If this were mine I would convert to cathode bias. Easy to do, at least on paper.
Just to note that removing the 220k local negative feedback resistors for the input stages will increase the stage gain a lot, from about 220/100 = about 2 (6dB) to the open loop value of about 30 (30dB).
That might cause the 2nd stage to overdrive.
To accommodate that increased gain, a volume control could be moved between the input and 2nd stages.
Replacing the 22k and 47k fixed potential divider resistors, or perhaps just the 47k.
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Pedal friendly simply means the amp sounds good with pedals plugged into the input. If this were mine I would convert to cathode bias. Easy to do, at least on paper.
Just to note that removing the 220k local negative feedback resistors for the input stages will increase the stage gain a lot, from about 220/100 = about 2 (6dB) to the open loop value of about 30 (30dB).
That might cause the 2nd stage to overdrive.
To accommodate that increased gain, a volume control could be moved between the input and 2nd stages.
Replacing the 22k and 47k fixed potential divider resistors, or perhaps just the 47k.
After reading your posts I am reconsidering modification and also given it's a pcb it's not that straight forward implement or reverse.
What is the technical reason for the input configuration to be regarded as not pedal friendly or is this an opinion based on anecdotal perception?
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What is the technical reason for the input configuration to be regarded as not pedal friendly or is this an opinion based on anecdotal perception?
All else being equal, a cathode biased input stages can accommodate a somewhat higher signal level (eg out of a pedal) before becoming overdriven than a grid leak biased stage.
But despite the grid leak bias, your input stages can accommodate large signal input levels, due to the inverting feedback amplifier configuration (there's a 'virtual earth', a near zero signal level, at the V1 grids).
Liking the amp as it is seems to largely remove any potential benefit from reconfiguring it.
https://mail.aikenamps.com/index.php/designing-single-stage-inverting-feedback-amplifiers
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Thank you for that concise explanation
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This may be off topic or it could be of some value to Yosser.
I generally understand the virtual earth mixer but I do have a question that someone may be able to answer. How does the 500k input potentiometer affect the input impedance as far as the gain of the mixer? I know it is a voltage divider but if it is set to 1/2 does the 250k to ground or 250k from the input jack change the gain of the mixer?
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... How does the 500k input potentiometer affect the input impedance as far as the gain of the mixer? I know it is a voltage divider but if it is set to 1/2 does the 250k to ground or 250k from the input jack change the gain of the mixer?
If the volume pot wasn't there, the input impedance would be 100k.
With it there and set to full CW, it's 100k // 500k = 83k.
Set electrically halfway, I think it'll be something like 250k + (250k // 100k) = 250k + 71k4 = 321k.
The signal 'gain' between pot input and output will be 71k4/321k=0.222, and the signal gain from pot output and the input stage anode will be 220/100 =2.2.
Total gain between input socket and input stage anode will be 0.222 x 2.2 =0.488.
Hope that makes sense, and isn't rubbish :icon_biggrin: