Gorilla TC65 amplifier - UK Vintage Radio Repair and Restoration Discussion Forum

MotorBikeLes · 15th Jan 2017, 12:00 pm

My pal picked this up a few years ago and has used it as a PA amplifier quite successfully, but its controls are very noisy. Apparently very common, and he has tried to clean them. I hope to sort it and likewise use it for PA, certainly nothing else. It has one control marked "Tube Cruncher" which it seems is gain (preamp gain?), a master vol, three tone controls (trb, mid, bass) and a reverb. I see no need for reverb, and wonder if I can simply replace the three tone controls with two fixed resistors each, bypass the reverb, and then only replace the gain and master vol with a couple of good quality pots from my old TQ stock, proper controls. I can't find a manual and have never played with anything beyond normal domestic radio/HiFi amplifiers. All advice gratefully received, especially a link to a circuit.
Les.

Herald1360 · 15th Jan 2017, 12:27 pm

Info from Dr. G is a bit sketchy, but it seems to be relatively well thought of.

What you propose sounds perfectly feasible, though it would perhaps be a pity to cripple the amp that way.

For the tone controls, check whether they're log or lin- that will make a big difference to resistors needed to set things flat. Generally feedback type controls have lin pots and passive types, log.

General control scratchiness can be exacerbated if the designer hasn't bothered to keep dc off the pots- sometimes a few extra capacitors can work wonders!

Some details about valves etc would help- it's unlikely the power amp design will be far off from whatever the valve manufacturers suggested in their application notes. A push pull output stage doesn't vary much apart from ultralinear (unlikely in a guitar amp) and fixed or cathode bias. Driving it offers more variations from simple concertina to differential, paraphase etc with or without cathode followers to drive the output grids.

Anyway, have fun and how about a few pictures of the innards?

MotorBikeLes · 15th Jan 2017, 1:20 pm

I have yet to open up the amplifier section, but is is clearly solid state. No room for valves in there. I would assume that measuring pot resistances at mid point would generally make sense, though maybe if for PA only, I would set treble at say three quarters and bass at one quarter before selecting the two fixed resistors. For the reverb, I would simply short out the "thing", a spring or similar I assume.
As I wrote earlier, I have never played with power amps, so anything "not obvious" will be a trap for me. And I don't want too many s/c three legged fuses.
Les.

Radio Wrangler · 15th Jan 2017, 1:56 pm

If it's running enough to show that the pots are scratchy, then the power amp section is probably chugging along happily and doesn't need any attention. The reverb could be a CCD chip or a spring. If you want it for PA only, I don't suppose you want to sound like a CB echo-mike so it doesn't matter.

David

paulsherwin · 15th Jan 2017, 3:45 pm

Have another go at cleaning the pot tracks.

jamesperrett · 16th Jan 2017, 1:18 pm

Remember that this is a guitar amp so the frequency response and tone controls may well have been tailored to giving a pleasing sound from a guitar. The preamp will also have a high input impedance which will alter the frequency response of many mics. Yes, it will work as a PA after a fashion, but nothing like as well as a proper PA amp.

15th Jan 2017, 12:00 pm	#1
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Gorilla TC65 amplifier My pal picked this up a few years ago and has used it as a PA amplifier quite successfully, but its controls are very noisy. Apparently very common, and he has tried to clean them. I hope to sort it and likewise use it for PA, certainly nothing else. It has one control marked "Tube Cruncher" which it seems is gain (preamp gain?), a master vol, three tone controls (trb, mid, bass) and a reverb. I see no need for reverb, and wonder if I can simply replace the three tone controls with two fixed resistors each, bypass the reverb, and then only replace the gain and master vol with a couple of good quality pots from my old TQ stock, proper controls. I can't find a manual and have never played with anything beyond normal domestic radio/HiFi amplifiers. All advice gratefully received, especially a link to a circuit. Les.

15th Jan 2017, 12:27 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Gorilla TC65 amplifier Info from Dr. G is a bit sketchy, but it seems to be relatively well thought of. What you propose sounds perfectly feasible, though it would perhaps be a pity to cripple the amp that way. For the tone controls, check whether they're log or lin- that will make a big difference to resistors needed to set things flat. Generally feedback type controls have lin pots and passive types, log. General control scratchiness can be exacerbated if the designer hasn't bothered to keep dc off the pots- sometimes a few extra capacitors can work wonders! Some details about valves etc would help- it's unlikely the power amp design will be far off from whatever the valve manufacturers suggested in their application notes. A push pull output stage doesn't vary much apart from ultralinear (unlikely in a guitar amp) and fixed or cathode bias. Driving it offers more variations from simple concertina to differential, paraphase etc with or without cathode followers to drive the output grids. Anyway, have fun and how about a few pictures of the innards? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

15th Jan 2017, 1:20 pm	#3
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Gorilla TC65 amplifier I have yet to open up the amplifier section, but is is clearly solid state. No room for valves in there. I would assume that measuring pot resistances at mid point would generally make sense, though maybe if for PA only, I would set treble at say three quarters and bass at one quarter before selecting the two fixed resistors. For the reverb, I would simply short out the "thing", a spring or similar I assume. As I wrote earlier, I have never played with power amps, so anything "not obvious" will be a trap for me. And I don't want too many s/c three legged fuses. Les.

15th Jan 2017, 1:56 pm	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,880	Re: Gorilla TC65 amplifier If it's running enough to show that the pots are scratchy, then the power amp section is probably chugging along happily and doesn't need any attention. The reverb could be a CCD chip or a spring. If you want it for PA only, I don't suppose you want to sound like a CB echo-mike so it doesn't matter. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Jan 2017, 3:45 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Gorilla TC65 amplifier Have another go at cleaning the pot tracks.

16th Jan 2017, 1:18 pm	#6
jamesperrett Octode Join Date: May 2007 Location: Liss, Hampshire, UK. Posts: 1,873	Re: Gorilla TC65 amplifier Remember that this is a guitar amp so the frequency response and tone controls may well have been tailored to giving a pleasing sound from a guitar. The preamp will also have a high input impedance which will alter the frequency response of many mics. Yes, it will work as a PA after a fashion, but nothing like as well as a proper PA amp.