...Baby tube Guitar amp...

[Joel]

James,

The best low-wattage guitar amp I've ever played is my version of a tweed Fender Princeton that I built. One 6V6 single-ended into a Hammond 125ESE, about 4.5 watts. It's very clean up to about 3/4 of the volume control, then it has a smooth, old fashioned break up. Very good note definition too.

A similar "project" is posted on the Angela Instruments website. But, I would suggest adding a 15 Henry choke after the rectifier, like I did. I also used a 6SL7 instead of a 12AX7. It's a lot more clean and defined than a stock Princeton, but if you want it to be dirtier you can use a smaller output transformer.

Other modifications from the original circuit include taking out the horrible tone control network and replacing it with an old Gibson style one - two coupling caps, a 0.1uF and a 750pF, with a 1 meg pot to switch between the two. Works great.
I hate the marshall/fender "tone stack" too. It can't boost the mids at all, and has very little usable range due to the steep slope.

Joel

[James D.]

Joel,

Thanks for your reply... a baby Princeton is an interesting idea. I'll check the reference...

Changing the 12Ax7 sounds like a good idea...and chyanging the tone stack is a must! The Fender or Marshall tone stacks are horrible! Your suggestion of the Gibson is an interesting idea. I've never used a Gibson amp so I'll try your suggestion out.

Thanks

Jmaes

[bournville]

James,

Yes, I remember once trying to sort out an output tranny for the ECL80 - they were sold under the description of "suitable for a small output pentode" and were the same as used in battery tube sets - the primary impedence was about 10-11k!

The P/ECL82 seems a much easier option. Good luck on the project.

Originally posted by ShiFtY
Just in case its any help, a great source of parts for this kind of thing is old tube reel to reel recorders... There are lots around in junk stores, no-one wants em!

In the UK I occasionally pick up old recorders at 'car boot' sales - usually about £5 ($8) a time, and they often contain a range of useful audio valves in good condition - ECL82s, EL84s, EF86s and ECC83s being the most common found, and Mullard the most common brand.

[Fuling]

I made a VERY small practice amp a while ago as a gift for my brother in law. I used a single ECC81 (12AT7) using one half as preamp and the other as "power" amp. The output trannie was from an old radio and had a 7k/4 ohm ratio. The speaker was a 4" 16 ohm fullrange unit.
Unfortunately the gain was a bit low so I had to add a SS diode clipper to get some decent heavy metal distortion.

Maybe not the best guitar amp ever, but fun as hell to build, especially as it didn´t cost me a thing!

[James D.]

Neat!

I did a similar thing with EF86 into ECC82, a pair of 1N914s and a 6" by 4" ellipitical speaker... it was great fun but didn't really challange a proper guitar amp! I did try it into a cab but it really wasn't good enough...

ciao

James

[bournville]

...I remember some weeks ago reading an article in an old radio mag which made great claims for the PCC88 rf double triode in small pp output stages. Originally designed for work in TV tuners, in pp mode, with fixed bias of -6v and a 200v on the anodes, output of 1.5w can be obtained, and at its maximum ratings, with fixed bias of -5v and 210v on the anodes, output goes up to 2.5w. The output tranny had a 10k anode to anode impedence. It could thus rival a SE P/ECL82 amp which is typically used in circuits giving 2.5w out, and is certainly more powerful than an ECC82 used in pp.

The heater is more of a problem though, being 7.6v...

[James D.]

Now that is tempting! And one could overdrive the power stage nicely too...


Does anyone have a view on triode mode versus pentode mode for a guitar amp?

How about DHT (31?) for a guitar amp?

[Joel]

Originally posted by James D.
How about DHT (31?) for a guitar amp?

Not recommended. They are too microphonic, IMO.
And, I have to think that any benefit of using a DHT would be fruitless with such a low bandwidth source as a guitar.

I have wanted to do an all triode guitar amp though! Yum. Probably the best bet is something like PP 6L6's in triode mode. 3 watts or so at 2% distortion.

[James D.]

DHTs

They are too microphonic, IMO.

Good point! So no DHT combo

I'll forgive you for liking 6L6's - the EL84 is better

Actually I like both in guitar amps a lot.

There is something nice about all triodes for good clean tones - but don't you think pentodes are nice for overdriven rock tones?

[Joel]

Originally posted by James D.
...but don't you think pentodes are nice for overdriven rock tones?

As I said, my favorite guitar amp uses a single 6V6 in pentode mode. I don't use many overdriven rock tones, but for clean jazz and country stuff it's the best.

I think people who want crazy overdriven distortion at bedroom volume levels need to really consider amps with a Master Volume control. Otherwise you need to limit yourself to an amp that hits the wall at milliwatts of output.