6BQ5/EL90 guitar amp schematics wanted

[PRNDL]

I recently purchased an old Dynavox amp with 2-6BQ5 in push-pull, 12AX7 preamp and 6X4 rectifier.

I'd like to convert it into a guitar amp and was wondering if anyone had a suitable schematic to follow.

I've searched and found that one version of the 1965 Gibson Skylark GA-5 has this exact tube lineup, but I can't find a schematic for it.

Thanks!!

[aletheian]

The 6BQ5 for your purposes can be considered an EL84, and the rectifier has no bearing on the rest of it, so you could easily convert it into any amp with a single 12ax7 and a pair of EL84... that should throw open the door of possibility a bit.

[Colt45]

PP EL84 output is good.. you'll probably need more than one 12ax7 worth of gain though, as guitar pickups don't put out much.


you could do something like a vox AC-15.. just skip the tremolo stuff.

You'd just need to add an EF86.

heres a looow res schematic

http://www.drtube.com/schematics/vox/ac15fact.gif

[PRNDL]

Thanks for the tips!!

Here's my choices:

Vox AC15 (without trem): has an EF86 preamp (1 stage), no tone control

Gibson GA-15 has a two-stage preamp (ECC83) with tone control.
I could use 12AX7's instead of ECC83's

Matchless Lightning uses two 12AX7's in the preamp. One in parallel, the other in cascode.

The Matchless Lightning must have huge amounts of gain! The power supply may be an issue. Also, the little chassis has a hole to mount only one more 12AX7.

I'm leaning toward the GA-15 over the AC15 because it has two preamp gain stages and a tone control. However, I believe the AC15 has many more fans.

Does the AC15 have enough gain for rock and roll distortion?
Is the EF86 preamp tube have much better tone than the 12AX7?

Thanks again!!

[ethermion]

Gibson has most of their schematics online. See
http://www.gibson.com/Files/schematics/GA-5T.jpg

It has 6eu7s instead of 12ax7, but you could figure that out.

I had a skylark, and I loved it. The GA-15 is a sweet amp, and that is what I would build, but I am a tele head... Ymmv.

I have access to a real deal ac-15. It is a nice amp, but not my cup of tea. It does not rock-and-roll for me.

As with all guitar amps, it depends on what you play and how you play.

[JPPP]

great topic, I have a similar problem too: an old mullard 5-10 (ef86, 2 ecc83 and 2 el84) taken apart, and since there is only one opt I could build guitar amp for my rock playing friends... only problem is I am a newbie when it comes to instrument amps... that gibson has the simplest psu I've ever seen; I did not know that you can do it like that , however my friends told me that the vox is a legend so maybe I'll try it first.

[aletheian]

I am thinking that a self-split output stage (that way you won't need a phase inverter) and two preamp gain stages might be a good topology.

[PRNDL]

I've put together a schematic based on what I'm familiar with (12AX7 preamp and phase inverter).

I'll be leaving the power supply and output tubes the way they are.

So far, it's up to $10 in parts, which isn't bad.

I do have some questions about the output transformer. It's currently wired for two speakers, but I don't know what impedance. The seller stated the amp sounds fine with 4 or 8 ohm speakers. I have a Mod15-6 4-ohm and a Jensen Trace Elliot 5" 8ohm speaker to try. Both were for little amps.

The output transformer itself has two pairs of wire - a cloth covered + and bare wire that is grounded. The bare wires are unusual. I suppose I could try the speakers I have and see, but I thought I might ask first.

[Wavebourn]

A first, decrease capacitances of coupling capacitors to 1'st grids of output toobs so when you overdrive it it won't "fart". Second, add one more tube and a Fender tone stack (the 1'rst stage and a stack may steal from Fender amp schemo).

Nothing more, and you will be satisfied!

[PRNDL]

I wired the amp using what the components in it (and a few left over parts) and it now works great!

The speaker transformer is the next issue. It's has two pairs of wires with one ground and the other going to the speaker. I measured 1 ohm for each speaker, and 2 ohms between the two speaker wires. Does this mean that it's wired for two 4-ohm speakers, and I can put a single 8-ohm speaker connected to the outer wires (removing the ground and putting it to the speaker -)?

I'd like to add a Baxandal tone stack with the values used in the Duncan tone stack simulator, but am without a hard drive for the PC.
Can someone look up the values (R5-9 and C3-6 in the schematic)?

Thanks!!