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Full Version: 5 Watt Tube Guitar Amp Conversion Project
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Here's a little 5 Watt tube rehearsal/ studio guitar amp that I converted from a 1960s tape deck for a few friends. It's been evolving over the past year based on their feedback.

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.
Hi NCtubes,

I'm all about fun projects and wacky prototypes but I worry about your use of cardboard - a disaster waiting to happen.

Was it meant to be removed after shipping?
(02-18-2021, 06:19 PM)DCarrer Wrote: [ -> ]Hi NCtubes,

I'm all about fun projects and wacky prototypes but I worry about your use of cardboard - a disaster waiting to happen.

Was it meant to be removed after shipping?

Yes, it is temporary,for use to protect the unit during shipping and the user until the unit is installed into a permanent cabinet. A guitarist/woodworker friend of mine has recently been making really nice cabinets for them. 

2 cabinets cost me more than a stereo tape deck with the 2 amps that I modify to make these.
Great to hear - I figured as much but had to say something in this age of Gorilla Glue.

How has your luck been finding good EF86s?
(02-18-2021, 06:29 PM)DCarrer Wrote: [ -> ]Great to hear - I figured as much but had to say something in this age of Gorilla Glue.

How has your luck been finding good EF86s?
Actually, I have been using the original tubes from the 1960s Akai / Roberts tape decks. I think they are all NECs and I haven't had a bad one yet.Akai had even installed rubber vibration collars on them. They've all been strong and quiet.
The cab looks good - worth the price I suppose given the value added.

I'm very happy to hear that you've had better luck than I with the EF86! A wealthy client once insisted that I build him an EF86 based amp. At first I declined due to the typical issues with that tube. A few weeks later he showed up with about 50 various samples. I kid you not - I tested every single one and found only 8 that were well behaved enough.

In the end the client was happy, I had two additional units to sell (I can't seem to build one amp at a time) and I had a few spare good EF86s for repairs.
Nice. Was microphonics the main problem with them?
(02-18-2021, 10:31 PM)NCtubes Wrote: [ -> ]Nice. Was microphonics the main problem with them?

Microphonic and excessively noisy samples were a major cause of grief. Some were "ok" with a couple of o-rings.

I haven't experimented much with cascoded triodes but certain configurations are said to have a pentode-like response without the pentode problems. There is a chapter in the Blencowe preamp book.
This was my first experience with pentode preamps and I am not running into any noise differences between the triode or pentode inputs. Beginner's luck I guess. Hope I haven't just jinxed myself.
Another short sample:
https://youtu.be/4vVNFjWatKU
Here are some cabinets that a friend of mine has been making for the amps.

https://www.dropbox.com/sh/06vxqk8zecz4j...VQO8a?dl=0