...Baby tube Guitar amp...

[James D.]

My son wants a low output practice guitar amp. It needs superb clean tones and great crunch but no fancy high gain stuff as he has plenty of pedals for that stuff...

So what to build? Any suggestions?

My inital thoughts are to use two ECL80's witha EZ80 rectifier. The two triode stages should have enogh gain with a tone stack inbetween (propbably Steve Bench's design) and run the output stage push pull triode or pentode... The conventional design would be switchable but I've been toying with using a pot. to vary the triode to pentode percentage... Has anyone tried this or have a view on how it would work - or wouldn't work! see drawing below...

Any suggestions or thoughts would be most welcome.

ciao

James

[bournville]

Are you sure you want to use ECL80s? The gain of the triode section is quite low and even in pp operation I doubt they would give more than 3w and the anode load is quite high, although the power consumption would easy be handled by an EZ80. I can recall only ever seeing one circuit using ECL80s in pp for a gram amp.

Did you mean ECL82s or other triode/pentodes by any chance? I ask because the ECL80 has a common cathode, yet your schematic shows different cathodes for the triode and pentode.

[SHiFTY]

I made the AX84 (www.ax84.com) using a 6X4 tube rectifier which was ok, but didn't have the "right" tone for me. The 4W or so that it creates is plenty of power though! I can't turn it up to distortion, its too loud for my room/the neighbours.

If you get a big guitar speaker, they can be 100dB efficient or more, which means that even 1W will be pretty loud. I have heard of people using a 12AU7 in PP as an output tube for 1W, apparently its quite good! (and distorts at usable volumes I would guess.)

BTW I am currently building a copy of a Fender Deluxe PP 6V6 for a friend (first paid project! )

[James D.]

Hi Bournville,

I was thinking of using it fixed bias - with cathode grounded and the ECL80 is lower power than the 82... I am only looking for about 1Wish in triode mode and a bit more in pentode is OK... Do you think this would work?

Using the ECL82 is a definite possiblity (actually PCL82 as I can get them for 3euros)

For gain I would cascade the two triodes and use an IT for SE to PP... should be okay for clean - not sure for crunch yet (I guess I could add a pentode front end to take care of that...)

Most of the time on clean it will run at about 100mW or less into a 12" cab of 94dB/1W/1m sensistivity. It's a practice amp. My son has a 200W Hughes and Kettner system for gigging that is incredibly loud but a bit hard to tame in his bedroom

thanks

Jmaes

[James D.]

Hi Shifty,

I've spent time on the AX84 site and thought of a moonlight... but I want to design my own take on it

I had to try quite a few speakers before I could find a relatively low efficent one that had the right tone...

Congrats on the paid work! Long may it last!

ciao

Jmaes

[bournville]

I admit that it is many years since I've used these tubes, but I remember I did have problems with stability because of the common cathode, because the bias arrangements for the triode and pentode sections are different.

I have not used a fixed bias arrangement with these tubes so cannot comment on it - I used cathode bias on the pentode and grid bias on the triode (via a large grid leak connected between grid and cathode - I think I used 10meg!) with a large bypass cap on the common cathode resistor to get rid of any tendency to instability - 250uf or so.

Other circuits I've seen use NFB between the anodes to keep the circuit stable, but this reduces the gain, and with ECL80s there really is not a lot to play with.

I don't think that ECL80s had much of a run as audio amps in addition to their other duties in old TVs because the ECL82 came out shortly after their introduction. The most innovative circuit using a ECL80 I've seen had it as a complete TRF receiver, using the pentode stage as an RF amp, the triode as a leaky-grid detector, and then the pentode stage reflexed back as an output stage! I must ask though, that since a single E/PCL82 would give just as much output as a pp ECL80 circuit, and this is a practice amp, not a hifi one, why not use it?

[James D.]

Bournville said

I must ask though, that since a single E/PCL82 would give just as much output as a pp ECL80 circuit, and this is a practice amp, not a hifi one, why not use it?

Good question! I guess partially because I've using ECL82s so the ECL80 would be different! Partially because it gives lower power pp and if it's works I'll build myself one for my bass (so pp is better than se...).

I'm not wedded to the 80s so the 82 SE is a real possibility. I guess the OPT will cost more for the SE so that might tip the balance...

Thanks for your recollections on the 80 - fascinating stuff - particularly the TRF reciever - that's really nice!

ciao

James

[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!

You will get sockets, transformers, tubes, power cable and lots of stuff, sometimes even a funky tweed case!

Just a thought.

[James D.]

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!

Neat idea!!! Thanks.

James

[James D.]

Hi Bournville,

Having spent my lunch break running some calculations - I can see that you are completely correct about the low gain and high Zout of the ECL80. It would be diffcult to make a good guitar amp from them...

So back to good old P/ECL82s. Plenty of gain in the triode sctions to drive a tone stack and get some 'crunch'.

I can run a relatively high Z OPT to reduce the output power and ensure that I can crash the psu rails for extra crunch too...

Thanks for the advice - sometimes I do listen

ciao

James