2 x 6L6 amp conversion

Rock · May 5th, 2008, 09:30 AM

I recently got a broken amp including two speaker cabinets for a bargain and now I'm trying to figure out what to do with it. It's a Geloso G27 that's been heavily modified (extra tubes and channels) and I think some of the original tubes have been changed in the preamp section. The ones I have is two 6J7, one 6SN7 and a 6N7. What i'm looking for is to simplify the design for example using a tube as a phase inverter instead of the transformer. All help appreciated.

Cheers

bradpdx · May 5th, 2008, 12:48 PM

Wow, what a strange amp design! Does this schematic reflect the original design, or the modified design?

This amp has so many design problems, I am not sure where to start.

1. Fixed bias derived from lifted center tap on transformer using 2 chokes - expensive and not adjustable.
2. Transformer phase splitter with lots of compensation to avoid oscillation - yikes! That's a bad design with band-aids.
3. Tone control is treble CUT only.
4. Hugely wasteful resistive divider used for screens
5. Volume pot defines input impedance
6. No negative feedback from OT secondary (the other negative feedback appears to be related to oscillation prevention)

I would consider tearing out EVERYTHING and starting over with a simple Fender tweed style circuit. You may have to create a more clever negative bias supply, but otherwise you have transformers and tube sockets and little else.

Rock · May 5th, 2008, 01:05 PM

Yes, that is the original circuit. The modifications are even wierder with two additional volume controls, two speaker outputs some kind of selector switch to choose between four different inputs etc.

The design seems very strange to me as well and it was not what I expected to find. Maybe it's easier to get new stuff altogether. A tweed princeton seems like a project with the right level of difficulty.

I might not give up on this one entierly because just maybe the transformers might be useful.

bradpdx · May 5th, 2008, 01:18 PM

I would think more like an early tweed Pro design, with 2 6L6s in a self-biasing arrangement. You would only keep those transformers and toss everything else.

Scott S · May 5th, 2008, 02:30 PM

The volume control on the input (along with the 6N7's) is very reminiscent of old Gibson amps.

Nevertheless, I agree with the others that you ought to scrap the innards and put a better circuit in there. I'd keep it all-octal for sure -- if 6SC7s are hard to find, you could use 6SL7s.

- Scott

4 Strings · May 5th, 2008, 02:31 PM

Quote:

Originally Posted by bradpdx

I would think more like an early tweed Pro design, with 2 6L6s in a self-biasing arrangement. You would only keep those transformers and toss everything else.

+1

Build one and let us know how it sounds! Who knows, you might end up with a really nice amp. And if you don't at least you will not have spent too much on it and you're gonna learn a lot along the way.

Rock · May 6th, 2008, 02:35 PM

I started measuring the power transformer using a sinewave generator and well... I've got a bit of a problem. I can only seem to find the taps from between 110V and 220V and not between 0V and 220V. There's supposed to be six wires on the primary but I see only five. Must have done something wrong when stripping the chassi or maybe this is just a lack of knowledge from my side. Everyting is sorted out except that ONE wire which prevents me from using the transformer as is. Irritating!

May 5th, 2008, 12:48 PM	#2 (permalink)
bradpdx Tele-Holic Join Date: Jul 2006 Location: Portland, OR Age: 49 Posts: 549	Wow, what a strange amp design! Does this schematic reflect the original design, or the modified design? This amp has so many design problems, I am not sure where to start. 1. Fixed bias derived from lifted center tap on transformer using 2 chokes - expensive and not adjustable. 2. Transformer phase splitter with lots of compensation to avoid oscillation - yikes! That's a bad design with band-aids. 3. Tone control is treble CUT only. 4. Hugely wasteful resistive divider used for screens 5. Volume pot defines input impedance 6. No negative feedback from OT secondary (the other negative feedback appears to be related to oscillation prevention) I would consider tearing out EVERYTHING and starting over with a simple Fender tweed style circuit. You may have to create a more clever negative bias supply, but otherwise you have transformers and tube sockets and little else. __________________ ---------- Tech Geek and Sensitive Artiste String bender ordinare!

May 5th, 2008, 01:18 PM	#4 (permalink)
bradpdx Tele-Holic Join Date: Jul 2006 Location: Portland, OR Age: 49 Posts: 549	I would think more like an early tweed Pro design, with 2 6L6s in a self-biasing arrangement. You would only keep those transformers and toss everything else. __________________ ---------- Tech Geek and Sensitive Artiste String bender ordinare!

May 5th, 2008, 01:05 PM	#3 (permalink)
Rock TDPRI Member Join Date: Aug 2007 Location: Sweden Posts: 13	Yes, that is the original circuit. The modifications are even wierder with two additional volume controls, two speaker outputs some kind of selector switch to choose between four different inputs etc. The design seems very strange to me as well and it was not what I expected to find. Maybe it's easier to get new stuff altogether. A tweed princeton seems like a project with the right level of difficulty. I might not give up on this one entierly because just maybe the transformers might be useful.

May 5th, 2008, 02:30 PM	#5 (permalink)
Scott S Tele-Afflicted Join Date: Dec 2005 Location: SW CR IA US NA PE Age: 27 Posts: 1,162	The volume control on the input (along with the 6N7's) is very reminiscent of old Gibson amps. Nevertheless, I agree with the others that you ought to scrap the innards and put a better circuit in there. I'd keep it all-octal for sure -- if 6SC7s are hard to find, you could use 6SL7s. - Scott

May 6th, 2008, 02:35 PM	#7 (permalink)
Rock TDPRI Member Join Date: Aug 2007 Location: Sweden Posts: 13	I started measuring the power transformer using a sinewave generator and well... I've got a bit of a problem. I can only seem to find the taps from between 110V and 220V and not between 0V and 220V. There's supposed to be six wires on the primary but I see only five. Must have done something wrong when stripping the chassi or maybe this is just a lack of knowledge from my side. Everyting is sorted out except that ONE wire which prevents me from using the transformer as is. Irritating!