Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

[crusty philtrum]

18-20 months ago, I bought this 1970's Japanese Guyatone combo amp. It's one of their amazing silver-face Fender clones from that era. This one is the GA-1050 model, named 'Reverb Jazz', and is rated at 40 watts from a pair of 6L6s. Six preamp valves, reverb and tremolo, and a pair of 12" alnico speakers. I got it from a friend of mine who owned two of these and also their big brother that gives 80 watts from four 6L6s. I had previously worked on his other 40 watter and the 80 watt version, and discovered that not only do they look like Fender amps, but the circuits were almost identical and the build quality was high.

This Reverb Jazz model is mooted to be close to a Fender Pro Reverb circuit, and it is. However, there were some differences ... certain resistors were closer to the CBS-era specs and made this amp clean most of the way up, making it more like a half-powered Twin Reverb. The biggest single difference was that this amp has a solid-state rectifier whereas the originals used a tube. But regardless, this amp did sound very good in the classic s/f way.

Here's the amp as it was when i bought it ....

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

I really like this amp, but I knew I'd upgrade the components and eventually tidy up the appearance. Sonically, i had the feeling that it might be much more suitable for my purposes if it had a tube rectifier and the kind of response that would give. It wasn't long before i installed some better resistors and capacitors onto the primitive copper-track board that Guyatone used instead of the fibre board as used in the Fender amps.






A new set of CTS pots for the reverb channel ...




and a new set Fender knobs for both channels ...

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Having worked on both the 40 and 80 watt versions of these amps, I'd noticed that they used the same chasis for both ... my 40 watt chasis had four large holes for four power tube sockets, but of course mine was only using two sockets, so I had two spare holes where I could locate the rectifier tube socket. While i was working in this area of the chasis, I decided to add an IEC mains input socket and a second fuse-holder for the DC side of the circuit. There was also a 'courtesy power outlet' which i removed to give me another location for the second fuse holder. It was just a matter of opening up these holes to take the new hardware.




The new Hammond power transformer with the 5 Volt winding for the rectifier filaments had arrived, so it was time to get that physically installed onto the chasis ...






along with the new socket for the rectifier tube ...

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Then it was time to flip it over and get into the wiring (I'd also replaced all the filament wiring for all the tubes ... the white twisted pairs waiting in the corner near the transformer to be connected) 'Before' pics on the left, 'after' on the right ...

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

I replaced the bias adjustment pot with two new ones and wired it so that each power tube has it's own adjustment control, seen here from both the inside and outside of the chasis ...




And I replaced all of the filter caps ...






At this stage the amp was ready to be powered up and tested. In the process of upgrading the components on the board, I'd discovered a few resistor values that were different to the Fender Pro Reverb schematic and would have been the cause of the amp staying clean at all volumes. Of course i changed their values to the early Fender specs and knew that this was going to change the amp's gain structure to a more desireable and traditional performance, i.e. some warmth, fatness and breakup as the volume increased.

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Once I'd established that it was working well, i removed all the components of the Normal channel because that real estate and tube is going to become an all-tube effects loop that will use the front panel jacks and pots.

As much as i liked the stock alnico speakers, they weren't up to the job at higher volumes and greatly restricted the sound. I had this vintage pair of Etone speakers that were made in Australia in the 1960s and were in extremely good condition. I decided to give them a try.

Stock alnico speaker ...




The Etones were a tight fit, but they went in and i instelled a set of T nuts for them on the front of the baffle board ...

 

[jtougas]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

If you don't give us an audio sample, I will hunt you down for the mother of all purple nurples.

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

In the meantime, Gibson175's Fender Twin had arrived and I had to put this project aside for a while. But while working on his amp, i arrived at a point where i was waiting for some parts, so I fitted the Guyatone baffle board with new Fender grill cloth.




And then in a flurry of activity one afternoon, this happened ...








and then it didn't take long to get to this ...






One thing I hadn't considered was the location of the newly-installed IEC mains inlet socket, which gets covered up by the top left corner of the upper back panel. D'oH !! But it's not too bad, i just need to cut a little bit of that panel away. Hehe.

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

and of course finally i got to see what this part of the operation had been all about ...












I still have to get back in there to build the tube effects loop circuit, but the amp is running well and sounding great. It is now, component for component, a Fender Pro Reverb, the earliest circuit without the CBS intrusions. The Etone speakers sound great ... at lower volumes the pure Fender cleans shine through, and they have a Celstion-style crunch when the volume gets higher. All up, the amp sounds and responds wonderfully, and I can see why so many people sing the praises of the Pro Reverb, this amp can do so much and would be an ideal gigging amp alongside a drum kit. AT 40 watts, it sits perfectly in the zone for beautiful Fender cleans through to classic rock tones (like most Fenders, it seems to take pedals very well).

So what may have seemed like a crazy project, tearing apart an already good amplifier, has ended up giving me a fantastic amp that's only going to be even better once i get the tube effects loop built ... Im hoping for some really special sounds with the El Capistan echo pedal in there.

Sorry i can't do clips right at the moment, but they are scheduled along with a whole other bunch of demos i want to make as soon as my mic preamps are back on deck. But hopefully this has been of some value, or simply has some entertainment value, and it kind of fits alongside my thread about restoring Gibson175's Twin Reverb as these two projects kind of overlapped and inspired one another.

before and after ....




There was a certain irony about this project happening at the same time as the work on Gibson175's Twin .... his amp was becoming more 'Guyatone' with his new board being copied from this amp, while this Guyatone was becoming more Fender at the same time. Funny how things work out sometimes, hehe.

 

[Edgecrusher]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Thats just plain righteous!

 

[MetalManiac]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Fckin brilliant! I like the old handwound paper leaf wound, or wahtever you call it-iron, though- thats where the tone is.Oh but i see, you had to replace the PT to accomodate the rectifier tube. Okay. I think the output transformer is the main tone transformer anyway, right?

 

[Gearjoneser]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Awesome work, Crusty. I thoroughly enjoyed both of your restoration threads. This could work out to be a lucrative part time business, or even full-time.

I love the Guyatone to Fender transformation, and even the way cream tolex looks against the silver cosmetics.
If you have more tolex, you should do the back panels. Rather than cutting that top panel, you could flip it upside down and lower it accordingly.

Overall, I've always loved vintage Fender amps, and the Pro Reverb has become my favorite. The combination of the blackface circuit w/tube rectifier, mated to more efficient speakers is a beautiful thing.

I got lucky though....I ended up with a blackfaced 74 that was owned by Aspen Pitman, and it's last checkup was by a great tech, Greg Leon. It got sold to Aspen's cousin, the owner of my city's best BBQ restaurant, and then I came down like a hawk and grabbed it way below market value. It's become one of my favorites, and it's staying with me forever.

 

[MetalManiac]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

I heard a 74 Silverface twin once that didn't sound all that bad. g.

 

[Gearjoneser]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Reading comprehension Jerry...

Back to Crusty's restoration...I like the idea of turning your normal channel's jacks into a tube driven FX loop.
Your Strymon El Capistan will sound beautiful in the loop, especially since you're probably using the knobs as send and return levels. What will the other 2 pots do?

 

[MetalManiac]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Reading comprehension Jerry...

Linda. I'm Linda. He told me you thats its okay whatever you want to call me you can call me, but "jerry" is gone now, and he wont be back anymore.

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Fckin brilliant! I like the old handwound paper leaf wound, or wahtever you call it-iron, though- thats where the tone is.Oh but i see, you had to replace the PT to accomodate the rectifier tube. Okay. I think the output transformer is the main tone transformer anyway, right?

The rectifier tube needs 5 Volts for it's heater (filament). The transformer that was in it didn't have a 5 Volt winding (I'd hoped it might, it would have made the project a lot cheaper), so i had to source a suitable transformer that did. The one I've used is a Hammond, I didn't have the kind of money for Mercury Magnetics or anything exotic. But the Hammond seems to be doing a fine job.

You're right, the output transformer usually has a much bigger effect on what we hear; if the power transformer affects anything, it's more likely to be in the response of the amp (along with the associated power supply design and construction, the most noticeable probably being the choice of rectifier and size of filter capacitors).

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Awesome work, Crusty. I thoroughly enjoyed both of your restoration threads. This could work out to be a lucrative part time business, or even full-time.

I love the Guyatone to Fender transformation, and even the way cream tolex looks against the silver cosmetics.
If you have more tolex, you should do the back panels. Rather than cutting that top panel, you could flip it upside down and lower it accordingly.

Overall, I've always loved vintage Fender amps, and the Pro Reverb has become my favorite. The combination of the blackface circuit w/tube rectifier, mated to more efficient speakers is a beautiful thing.

I've mostly scaled back 'outside' electronics work now to concentrate on my own gear and music. Too much of this kind of work can become deflating, although i do enjoy building and maintaining gear for a few friends. Mostly i get a kick out of doing stuff for people like Gibson175 who I know will use the gear regularly and get the most out of it, and get great enjoyment from the process.

I was completely unaware of the Fender Pro Reverb until i got this Guyatone and then i seemed to start hearing about Pro Reverbs all over the place. In fact i'd seen Gibson175 singing their praises here a couple of times, and then I noticed you (GJ) mentioning something about owning one, again speaking very highly of it/them.

I have a big roll of the cream tolex, and have used it on my Matchless clone head and homebrew 2 x 10 cab, and now the Guyatone. I can't even remember where it came from, it just seems to have been amongst my stuff for the last twenty years, hehe. I didn't cover the back panels because they are a tight fit and probably need the wood trimmed down a little bit before i can re-cover them (i think this tolex is a little thicker and makes things fit together more tightly).

I'll have another very interesting build/rebuild/ coming up soon, I'm really looking forward to that one.

 

[crusty philtrum]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Reading comprehension Jerry...

Back to Crusty's restoration...I like the idea of turning your normal channel's jacks into a tube driven FX loop.
Your Strymon El Capistan will sound beautiful in the loop, especially since you're probably using the knobs as send and return levels. What will the other 2 pots do?

The 'Normal' channel input sockets will become the send and return jacks for the loop. Two pots will be the level controls, one each for the send and return. The bright switch will become the 'high/low level' switch for the effects return (i.e. pedal or rack unit). One of the knobs will become a rotary switch to select either parallel or series operation of the loop, with possibly an 'Off' position as well, that bypasses the loop entirely. The Normal channel's tube, both halves of V1, will become the buffers for the loop, and some components can be mounted on the board where the normal channel components used to live.

Because the Guyatone has a midrange control on each channel, that leaves one spare pot. I'm thinking about making that a variable negative feedback control, which could be a fun facility to have. The loop i'm going to build is the one from Kevin O'Connor and his London Power company ... the description and schematic is in K O'C's book 'The Ultimate Tone'. It is said to be very transparent and not affect the core amp tone. I'll find out for myself soon enough.

 

[Diminished Triad]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

What was the sound difference before with SS rectifier and now that you have a tube rectifier?

 

[jimijames]

Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)

Drool.

That's an awesome build crusty. Really admire the craftsmanship throughout. Bet it sounds killer.