5 Watt Tube Guitar Amp Conversion Project

[sll]

A 5 watt rehearsal/studio tube guitar amp converted from a 1960s tape deck.

https://youtu.be/4vVNFjWatKU


https://youtu.be/Ya2bgr9mEtM

This amp was designed and constructed using the basic foundation of a 1960s (Akai M8) Roberts 770X tube reel to reel tape recorder. The amplifier circuitry has been optimized to specifically serve as a guitar amp rather than as hi-fi equipment. Where practical the original vintage components, including the tubes, have been kept.

There are 2 instruments inputs which share the common volume and tone controls. The 2 inputs however employ separate, and different, input stage tubes and therefore sound very different. The first input uses an EF86 (6267) tube. These tubes were used in 1960s vintage VOX amps, and are still used in some Dr. Z, Matchless, and other boutique amplifiers. The 2nd input uses a 12AX7 (12AD7) tube. 12AX7s are the most common preamp tubes used in guitar amplifiers such as Fender, Mesa Boogie, and Marshall. All of the tubes are the original vintage 1960s tubes. This amp is very quiet (background noise and hum) and breaks up nicely at reasonable volume levels.

The tone circuit uses a modified passive Baxandall/James circuit (Bass and Treble controls) similar to the type of tone stacks used in the original Ampeg, Orange, and some Dr. Z amps. Additionally, a boost switch increases the gain and frequency response of the amp providing additional tonal flexibility.

The output stage is a single-ended class A power amp. This amp uses an EL84 (6BQ5). Single-ended class A amps were used in many lower power 1950s-60s Fender Tweed and Blackface amps and in Vox's AC-4, which also used the EL84 tube. The amp uses a 6X4 tube rectifier. This amp works great for rehearsal, small venue use, and especially studio recording.

[ChopSauce]

Interesting ...

Any schematic?

There are so many variations around the EF86 tube / vintage Vox-JMI AC4 circuit ...

Were you inspired by Merlin Blencowe's revision of the EF86 seetings or did you instead manage to copy bits of existing circuits?

[sll]

Sorry, don't know who Merlin Blencowe is. I got ideas from many different sources and self calculations. PM me with your email address and I will be happy to share schematics of a couple of versions I have used.

[thetragichero]

you're more than welcome to share them here. since there ain't nothing new in terms of a relatively "ancient" technology like tube amplification (heck, some of the most famous amps had entire sections lifted straight from tube manuals) there's no real intellectual property to be worried about so us diy folks freely share that kinda stuff, often asking for tweaks from the community at large. without fine folks like merlin blencowe and rg keen (among many others) dedicating their time to share their knowledge, i wouldn't understand nearly as much as i do right now

[sll]

Here is one of my favorite iterations. Note that the 30 ohm resistor on the output safety switch should not be there, the 8 ohm resistor goes directly to ground. There may be some other errors on the drawing but I don't have access to my latest files right now.

[ChopSauce]

Thanks!

See: http://www.valvewizard.co.uk/pentode.html for a short version of Merlin's insights. His books contains more details, though.

With the help from many members of a french forum to realise a circuit which incorporates most of his recommendations: https://ampgarage.com/forum/download/file.php?id=57968

The outcome has a "real" tone stack and a master volume, which proves usefull for bedroom use.

[pdf64]

...With the help from many members of a french forum to realise a circuit which incorporates most of his recommendations: https://ampgarage.com/forum/download/file.php?id=57968

That looks interesting, but what’s the ‘aux’ input intended for, and why such a high value for its 330k grid stopper?

[ChopSauce]

I've been using it as an output for the soundcard of a desktop computer. The plan was to have an aux input as the Fender Greta. Afterwards I think a second channel would be a (far) better use for the second triode. That or using the pentode as an OD tube, "à la" tweed overdrive special.

[pdf64]

I've been using it as an output for the soundcard of a desktop computer. The plan was to have an aux input ...

Just to note that the 330k grid stopper will act as a low pass filter, corner freq about 4kHz.

[ChopSauce]

Yes, thanks a lot!

I should have mentionned that I published the schematic to keep a trace of the current state of the experiment. Plus what I am rather pleased with rather is the "promise", than the result; so far.

Another point is: as is, the EF86 seems to be pushed a bit hard when I plug in a guitar with humbuckers.

[sll]

I had wanted to provide 2 very different inputs so I had used the 12ax7 as the mixer stage and the 12ax7 and the ef86 as the 2 input stages. The range of tonal complexities across different pickup types has served very well in different performance scenarios.

[ChopSauce]

Sure!

I also noted the 250k anode resistors, which doesn't seem that much of a common value.!?

[sll]

Yeah, they are remnants of the original Akai circuitry. They were working okay for the sonic results I wanted and were incredibly quiet so I left them and the cathode resistors that came in the units. I've always been a believer of "if it ain't broke don't fix it". Here's another example of the different tones it can produce.

https://youtu.be/4vVNFjWatKU

[Phil_S]

You've got *real* chops. Very nice amp. Great demo. I don't think you said what speaker and speaker config.

[brewdude]

That sounds great