Amp iron recycling project - what to build?

Keefy

Some while ago I acquired the battered remains of a Carvin Vintage 33 combo - no speaker, no valves, but the PT and OT both work. I have stripped out the rest of the guts and saved anything useful, including the chassis of course. The 1x12 cab is somewhat roadworn but I have managed to remove the worst of the soiling.

The original circuit used four EL84 power valves, 2 pairs in push-pull like an AC30. The schematic can be found here and shows working voltages.

I have tested the OT using the guidance in Uncle Doug's video, and found that the 4ohm and 8ohm speaker taps have impedance ratios of 1,000 and 500 respectively. (The 16ohm speaker wire has been clipped off very close to the transformer and is probably unusable unless I start to disassemble the OT.)

My building/modding/repairing experience includes a handful of tranny and valve amps and quite a few pedals. Armed with a pdf copy of the RCA Receiving Tube Manual, I should be able to work out what output valves would work with the transformers, and at what HT voltages. The PT has no 5V tap for a rectifier, but I could add a small extra transformer for this if I wanted.

But what to build? I already have a Matchless Lightning (half an AC30?), a 5E3 Deluxe clone, and a Princeton Reverb '68 RI, and I'd like to make something that's different enough from these to justify the effort. I'd quite like to include reverb (already got a couple of spare tanks) but that's not a deal-breaker.

So far, my candidates include:

• 5A3 Deluxe (probably no reverb for this one!)
• A clean Dumble-inspired circuit using two 6V6 or two 6L6
• An old Supro of some kind.

Any others I should consider?

For inspiration, here is the chassis before and after gutting:

Keefy

Currently thinking of turning this into a Deluxe Reverb kind of amp - maybe only the effects channel.

The DR circuit uses a separate tapping to produce -49V DC for biasing. I don't have that on my PT, but I notice that the Carvin generated its negative bias voltage from the 260V AC secondary. Fortunately the DR's tremolo acts before the phase inverter, and not on the power valves, which should make it easier. I just need to find a suitable bias circuit.

Google here I come...

ICBM

Trainwreck Liverpool 4-EL84 with reverb.

Keefy

ICBM said:

Trainwreck Liverpool 4-EL84 with reverb.

Ooh thanks, I'll check that out...

ICBM

Keefy said:

ICBM said:

Trainwreck Liverpool 4-EL84 with reverb.

Ooh thanks, I'll check that out...

I should have said that you would need to insert the reverb section from something like a Fender Twin into the middle of the circuit since they didn't come with it, but that would still be possible with the number of valve cut-outs you have.

Keefy

ICBM said:

Keefy said:

ICBM said:

Trainwreck Liverpool 4-EL84 with reverb.

Ooh thanks, I'll check that out...

I should have said that you would need to insert the reverb section from something like a Fender Twin into the middle of the circuit since they didn't come with it, but that would still be possible with the number of valve cut-outs you have.

I won't be using those cut-outs, I'll have to make some new ones. As you can see from the photo, the original amp had all the valve sockets mounted on the PCB, which was in the front half of the chassis (as all the pots were board-mounted too.) The new cut-outs will be in the rear half of the chassis base, between the transformers. Therefore if I can keep the valve count down, so much the better - otherwise I might need to move the transformers forward, and the OT needs a rectangular panel cut-out.

The chassis is 14-gauge steel. This is slightly too big for sheet metal punches, which claim to be good for 'up to' 16-gauge. I'm going to have to find a way to lash up my drill stand to my workmate, and then clamp the chassis while I have at it with a step drill...

What I do know is that the PT supplies enough heater current at 6V (or 5.8V according to the schematic) to run four EL84's and four 12AX7's.

If the Liverpool is an AC30-type design that might be too close to my Matchless Lightning.

So, still looking at the Deluxe Reverb, but probably with a SS rectifier like the Carvin had. I've been looking at the original '65 schematic as well as the RI. The original is easier to follow! I've realised that I just need to use the right resistor values drop the negative bias voltage into the right range.

ICBM

Keefy said:

I won't be using those cut-outs, I'll have to make some new ones. As you can see from the photo, the original amp had all the valve sockets mounted on the PCB, which was in the front half of the chassis (as all the pots were board-mounted too.) The new cut-outs will be in the rear half of the chassis base, between the transformers. Therefore if I can keep the valve count down, so much the better - otherwise I might need to move the transformers forward, and the OT needs a rectangular panel cut-out.

The chassis is 14-gauge steel. This is slightly too big for sheet metal punches, which claim to be good for 'up to' 16-gauge. I'm going to have to find a way to lash up my drill stand to my workmate, and then clamp the chassis while I have at it with a step drill...

Honestly... I think you're making a rod for your own back and will end up with a less satisfying result. If you ditch the PCB and chassis-mount new sockets in those cutouts it will be much easier and neater than making new ones and leaving the old ones, even if you cover them.

If not an AC30-based 4-EL84 design, how about a 6G9 Tremolux with twice the power and a solid-state rectifier? That also has the right number of knobs if you put mid controls on both channels .

Keefy

ICBM said:

Keefy said:

I won't be using those cut-outs, I'll have to make some new ones. As you can see from the photo, the original amp had all the valve sockets mounted on the PCB, which was in the front half of the chassis (as all the pots were board-mounted too.) The new cut-outs will be in the rear half of the chassis base, between the transformers. Therefore if I can keep the valve count down, so much the better - otherwise I might need to move the transformers forward, and the OT needs a rectangular panel cut-out.

The chassis is 14-gauge steel. This is slightly too big for sheet metal punches, which claim to be good for 'up to' 16-gauge. I'm going to have to find a way to lash up my drill stand to my workmate, and then clamp the chassis while I have at it with a step drill...

Honestly... I think you're making a rod for your own back and will end up with a less satisfying result. If you ditch the PCB and chassis-mount new sockets in those cutouts it will be much easier and neater than making new ones and leaving the old ones, even if you cover them.

If not an AC30-based 4-EL84 design, how about a 6G9 Tremolux with twice the power and a solid-state rectifier? That also has the right number of knobs if you put mid controls on both channels .

Hmm... I will look again at whether I can fit a turret board or tag board between the pots and the existing valve holes, as per the usual Fender layout. The PCB is long gone by the way!

I've just looked at the Tremolux schematic and I see it takes its negative bias from the unrectified HT, so I can copy that part of the circuit anyway.

ICBM

You could easily get a single tag strip between the pots and the valve sockets, and another one or a two-row board on the other side between the transformers. You would need to work out your own layout but it wouldn't be too hard.

Speaking from experience, making anything other than small bolt holes in a chassis like that without using proper punches is a massive pain and never quite works as well as you think it will...

Keefy

ICBM said:

You could easily get a single tag strip between the pots and the valve sockets, and another one or a two-row board on the other side between the transformers. You would need to work out your own layout but it wouldn't be too hard.

Speaking from experience, making anything other than small bolt holes in a chassis like that without using proper punches is a massive pain and never quite works as well as you think it will...

Yes, I could always mount the bias and filter cap boards in the space between the valve sockets and the rear panel. Metalwork is not my forte! If I go for a two-channel Fender circuit I will probably ditch one of the channels anyway and just keep a 'vibrato' channel, with any bright cap on a switch, which will further simplify the layout.

Is there a schematic out there for a DR but with 6L6's? Or would that basically be a Vibroverb?

ICBM

Keefy said:

Is there a schematic out there for a DR but with 6L6's? Or would that basically be a Vibroverb?

A DR with 6L6s is a DR . The only change you might need is to increase the bias supply voltage slightly by changing the grounding resistor in the supply for a higher value, but in fact they usually bias OK with 6L6s within the range of the trimmer.

Even the output impedance is more correct for 6L6s than 6V6s with the stock 8-ohm speaker - my suspicion is that the amp was designed for 6L6s originally, but changed to 6V6s before it went into production, to save money... Leo was notoriously tight-fisted and wouldn't have used a more expensive valve than he absolutely had to.

Keefy

ICBM said:

Keefy said:

Is there a schematic out there for a DR but with 6L6's? Or would that basically be a Vibroverb?

A DR with 6L6s is a DR . The only change you might need is to increase the bias supply voltage slightly by changing the grounding resistor in the supply for a higher value, but in fact they usually bias OK with 6L6s within the range of the trimmer.

Even the output impedance is more correct for 6L6s than 6V6s with the stock 8-ohm speaker - my suspicion is that the amp was designed for 6L6s originally, but changed to 6V6s before it went into production, to save money... Leo was notoriously tight-fisted and wouldn't have used a more expensive valve than he absolutely had to.

There's my plan then - a 6L6-powered single-channel DR.

pintspiller

Fender Harvard?

Keefy

pintspiller said:

Fender Harvard?

Thanks but no, I'd want a bit more going on than that. As above, now more or less settled on a single-channel Deluxe Reverb with 6L6's.

PeteC

Following this with interest.  By far the most exciting fender circuit build I have done was a 5E5A fender pro using 2 6L6s.  That thing had harmonics all over the place and was a fantastic all round amp.  I later modded the chassis to a 1x12 silver jubilee combo which I still have but I am planning on building another 5E5A.   Check out the circuit - some really good things there.   The amp gets A good description in “Amps - the other half of rock n roll”.  I think it might be a Mitch Colby book ( or at least he was involved ).  If I had a blank chassis and iron I would build another in a heartbeat

to be honest I have been a bit underwhelmed by the deluxe circuits I have built.  I like the 5e3 but it can be awfully flabby unless modded.   

Anyway good luck with the build,and please post pics of your progress. 

Stay well......

Keefy

PeteC said:

Following this with interest.  By far the most exciting fender circuit build I have done was a 5E5A fender pro using 2 6L6s.  That thing had harmonics all over the place and was a fantastic all round amp.  I later modded the chassis to a 1x12 silver jubilee combo which I still have but I am planning on building another 5E5A.   Check out the circuit - some really good things there.   The amp gets A good description in “Amps - the other half of rock n roll”.  I think it might be a Mitch Colby book ( or at least he was involved ).  If I had a blank chassis and iron I would build another in a heartbeat

to be honest I have been a bit underwhelmed by the deluxe circuits I have built.  I like the 5e3 but it can be awfully flabby unless modded.   

Anyway good luck with the build,and please post pics of your progress. 

Stay well......

Cheers! You're right about the 5E3, it can make you reluctant to use the neck pickup.

Anyhow, first thing to do is work out a circuit. I will be dropping out a channel and modding the bias circuit. I think I will also incorporate a fuse in the HT line.

After that I need to draw up a tag board layout, then draw up a BoM. If only I was confined to my home with time to spare for this project. Oh wait...

Danielsguitars

I'm gassing for a bandmaster personally and I've got stuff kicking about but need motivation and more time, I'll be watching this unfold 

Keefy

Note to self: Don't sit poring over circuit diagrams last thing before going to bed. My brain has been trying to sort out impedance matching in a not-very-restful half-sleep!

I have worked out that the OT will give a plate load of 4k if I use the speaker taps as marked (which I understand is normal for a 4x EL84 setup), or 8k if I connect an 8ohm speaker to the 4ohm tap.

The PT puts out enough AC for a B+ of about 360V. If I am understanding the RCA manual correctly (p354 for those following at home), the load resistance for 6L6's in push-pull class AB at 360V plate is 3.8k, so I'm guessing 4k would be fine for the load. I can't find any specs for 6V6's in Class AB, just Class A.

I'm going to take some measurement on the OTs of my 5E3 and PR with a view to increasing my understanding of what's going on in those amps. Or at least I can copy something that works! It's not like anyone is standing over me to do this build - if you are following this thread, be prepared for a long haul...

ICBM

360V is much too low to give the correct B+ voltages for a Fender BF amp, even a Deluxe. It will work, but it will likely sound quite different.

Given this as well, I really think you’re better off using a 4-EL84 power stage. Then all you have to do is increase the voltages further down the chain.

Keefy

Thanks for that @ICBM you have doubtless saved me hours of wasted time there!

I’ve started looking at AC30 type circuits. I’m not sure I want three separate channels but I found the Wrocket on the @Modulus_Amps website which looks like a contender. It’s based on the Trainwreck Rocket. I need to track down a circuit for it, not least because the turret board layout is intended for a head and is the wrong way round for my chassis! For now I've downloaded the Liverpool schematic.

Modulus_Amps

Rocket or a Liverpool are obvious contenders, the rocket is a fab amp, I don't play mine enough really.

The liverpool will be closer in tone to a DR than the rocket, by that I mean the feel of a plate fed tone stack and gain stages after the tone stack, you could easily change the tone stack to blackface spec too and add reverb.