Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Martinb28 on August 18, 2016, 10:18:01 am
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What's up? I am new to this website because I am new to owning an EL34 amp. Finally picked one up off Craigslist and it's pretty cool. It is handwired and unlabeled. It has some interesting features:
- pair of EL34 powertubes
- Controls: Treble, Bass, Volume, Reverb
- a little red switch located between the 1st and 2nd preamp tube - seems to either enable another pre-amp gain stage, or functions like the "Fat" boost in a Blues Junior. Definitely a large volume boost, but not much (if any) added distortion
I have yet to determine what all the other tubes are, but it appears to have 5 12A?7 preamp tubes and a tube rectifier.
What other information would be helpful to determine what this is? It is in a homemade Tweed cab with a 10" AlNiCo speaker that is completely blue with no identifying marks.
A picture of the circuit is attached here and a link to a much larger picture is https://www.dropbox.com/s/b62go9xfyezs6qf/2016-08-17%2007.24.46.jpg?dl=0. (https://www.dropbox.com/s/b62go9xfyezs6qf/2016-08-17%2007.24.46.jpg?dl=0.)
Thanks!
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quick look says it's a boutique build :dontknow:
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If one figured two tubes for reverb & one tube for an LTPI (long tailed phase invertor), that would leave two more tubes and 4 triodes (presuming they are 12A_7 type dual triodes).
IF one of the tubes V2 is wired with a cathode follower, I would guess that it's some type of Marshallish/Bassman topology?
IF the V1 tube has paralleled wiring, then you'd have something with some similarities to a topology like the Texas Raptor but with EL34's?
I am just making a totally wild guess trying to guess how they would use up triodes on 5 tubes?
IF you can take a photo of the wiring on the first three tube sockets and report what kind of tubes make up those five tubes, we can probably make a more educated guess?
Since there are only two tone controls, I would guess that you might have either a Fenderish type with a fixed mid resistor instead of a mid pot ............ OR a James/Baxandall tone stack?
The reason I am guess the Texas Raptor type similarity idea over Fender AB763 is because with 5 tubes on a Fender, you'd probably have vibrato ........ OR a second volume control?
With respect, Tubenit
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When my amps go to public, they go with a PDF and hardcopy schematic, but sadly, musicians have a hard time keeping track of cables, let alone technical doc's :think1:
I'd just take the time and reverse engineer, or go originator hunting. Looks like a cool amp though
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Thanks for the feedback. I think it is some sort of Vox hybrid that combines the Normal Channel (EF86 preamp tube) with the Top Boost Tone Stack and adds reverb. I think this because it has an EF86 as the first gain stage (like the old school Vox amps did) and the Top Boost Tone Stack is only treble and bass. But my guess is not much more than a wild guess.
I looked at the tubes and think this is what they are:
V1 - EF86
V2 - 12AX7 / ECC83
V3 - 12AX7 / ECC83
V4 - 12AX7 / ECC83
V5 - CV4024 / 12AT7 / ECC81
V6/7 - EL34 / 6CA7
V8 - RCA rectifier of some sort
Reverb Tank - Accutronics 9AB2A1B - Made in USA
Speaker - Eminence Blue 10446 - ALNiCo
My guess is the Switch is determine pentode or triode for the EF86. That is common in Vox amps and supposedly would cause the large volume boost I hear.
Not sure what V2-4 do. I am not certain they are 12AX7s - the writing on the tubes are mostly faded. My guess is V2 is cascading preamp, V3 is the tone stack, and V4 is the reverb?
Anyone seen a design like this? Any ideas? I attached more pictures.
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That is a very good start, but the pics will likely need to be quite a bit larger to make out what parts connect to what pins.
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> what this is? It is in a homemade
1) Ask the seller?
2) Take it to the local guitar swap-and-hock, ask the oldest staffer if he has seen it before.
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Since it has a Vox-ish look-and-feel to it, a very good exercise for you to go through would be to get yourself a VOX AC30 schematic (or better yet a Matchless DC30 schematic - they are almost identical) and see if you can visually trace the signal yourself, comparing the components and values as you go. You will know pretty quickly if they match or are different.
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+1 Great way to learn and/or figure it out. :icon_biggrin: