| 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 |
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| 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. |
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| 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! :D) |
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| 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 |
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| 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 |
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| 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? |
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| James D. |
Bournville said
| quote: | | 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 |
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| 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. |
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| James D. |
| quote: | | 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 |
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| 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 :nod:
ciao
James |
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| 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 |
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| 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 |
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| 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.
| quote: | 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. |
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| 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! |
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| 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 |
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| 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... |
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| 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? |
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| Joel |
| quote: | 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. |
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| James D. |
DHTs
| quote: | | 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? |
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| Joel |
| quote: | 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. :bulb: :nod: |
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| James D. |
| quote: | | 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. |
Actually both my boys use a Boss GT6 if they want that.
But to do the hard crunch of late 60s rock you need to overdrive the power tube and OPT - a Master Volume control works before the power tube so no good - although it does work well on ss amps.
I'm trying relatively inefficent speakers to keep the volume levels down too...
Maybe I should use an EF86 as output!!!
James |
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| fdegrove |
Hi,
| quote: | | Maybe I should use an EF86 as output!!! |
Philips MINIWATT ?:clown:
Ciao,;) |
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| Arrrg it Bit me |
Joel,
which Gibson is your favourite, from which amp? |
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| James D. |
This last weekend I was asked to knock up a quick guitar amp for a friend and all I had othe shelf was a 6N6P and a matched pair of 6C45pi. I had a Ra-a 11K OPT and a small psu trannie so I was away...
I used the 6N6P as a twin triode pre-amp with gain and tone controls and used the 6C45s in PP LTP as OPT. B+ was 300V. It worked really well having a super clean tone with excellent response to fingering. The pre would push into overdrive starting to crunch up nicely and the output stage would also push through into overdrive (6C45s where biased at -2.5V and I had 16Volts of drive available before the master volume!). I'm a bit worried about how positive the 6C45s grid gets driven but it has survived several hours of abuse so far. These valves are tough!!!
I guess I should put a diode on it to limit positive drive to 0.7V as it is well behaved up to about that point but it is such a nice overdrive sound that I might leave it and see how long the valves last...
It's quite different from my sons 2A3 guitar amp or the ECL82SE guitar amp. It's closer to 60s Marshall sounds when overdriven...
I refer to the PAEng site a lot. Stefano really knows his stuff
ciao
James |
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| Bandersnatch |
Hey-Hey!!,
I am putting a simple one together for a friend of mine. Simple and easy. A 6Y6 as power tube, U-L connection, fed by a 6J4 with a DN2540 as input gain stage( no filament hum from that puppy, eh?). Power from a 261G6 Hammond and SiC diodes into a CLC filter, RC decoupled for the gain stages. Huge SE-UL hammond OPT and may run anything form 6K6,6W6 up to an EL34, though for reasons of economy the 6Y6 is likely going to be the final answer...
regards,
Douglas |
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| PRR |
Get a 1940s-1950s "5-tube" kitchen AM radio. VERY Preferably the kind with a power transformer, not one of the Hot-Chassis "AC-DC" affairs.
Ready-made, just cut the wire to the top of the Volume control, wire a jack to the top and bottom of that pot, and put a high-gain booster-pedal between guitar and jack. (Yeah, I know ready-made is no fun: but these antiques ARE cute and anyway there are lots of mods you cna do.)
They make 1 watt as single-ended pentode. The "better" ones might make more.
Add a triode switch for a less nasty tone.
They usually have a hefty treble-cut capacitor on the output transformer primary. That's needed to cut AM static but most guitarists like another octave or two of treble response. Try with that cap, try without that cap, and split the difference to taste. 1/2 to 1/4 the original value should be sweet.
The sensitivity at the top of the volume control is about 50-100 milliVolts, so you need another gain of 10 to get guitar-pickup sensitivity, plus enough to cover whatever losses your tone-stack gives. You can yank all the wires from the two IF cans to the tube between them, which is a Pentode, and re-wire as an audio voltage amp. Without tone-stack, it might work as Triode. With tone stack you will have to work it as Pentode (or bodge the RF/Conv tube into another audio stage). 100K on plate and 1Meg for screen should work. Cathode resistor will be -about- 1K; change that until the plate sits about 1/2 to 2/3rd of that stage's supply voltage. Or just yank all the wires and use a 6SL7. (If this is a MINI socket set: 6AV6 is 7-pin and sorta-like half a 12AX7 only different. However I think you'll be happier with an OCTAL set.)
Super-size the filter caps: 30µFd got the 1955 price down so your kid's grandma would buy it, but today's caps are so cheap you may as well go 100 or 220µFd everywhere. Typically the RF/IF stage ran on fairly raw power; add a 22K/47µFd decoupling section to clean-up the power to the low-level stages.
You can try this with the original 4-inch speaker: cute desktop amp. However if the kid rocks-out the full watt, the speaker may croak.
If you go with an larger external speaker, you may want to up-size the coupling and cathode caps 2X or 4X stock, to pass the bass that a larger speaker can handle and needs to justify its floor-space.
6EM7 is a classic US TV tube with a hi-Mu volt-amp and a low-Mu Power amp in one bottle, both triodes. It can push way over a watt of audio as SET within ratings. It is very available as NOS. I once sketched it as a 1/2 watt headphone amp, using dirt-cheap parts, but I think you'd be able to see how it would work as a speaker amp and get it up to a walloping Watt. Again it needs preamp gain, and surely two stages.
Your pot-sweep triode/pentode plan: it may barely work. The screen impedance may be 10K or less, so at mid-point on a 50K control the screen may feel isolated. If you reduce the pot to give the screen a lower source impedance, it steals significant power from the load. Since this is not a power-critical application, that may be fine. But I suspect you will have to play with it to find an optimum set of values. If you could find a center-tapped winding (oversized P-P tranny?) you could do a 3-way Triode UltraLinear Pentode selector, which is probably plenty of mode-control. |
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