You might try 1000pF or so across one of the pre amp plate resistors (R19 or R5) to kill the RF gain. Some say Marshall does this to avoid HF feedback from the front to back end.
I have used 6DX8/6F4P valves in a number of small guitar amplifiers. Circuit layout seems to be very important to avoid instability with these valves. I found it necessary to use larger than usual grid stoppers - as you have done - on both the triode and pentode sections, and to have them mounted right at the valve socket.
I agree with PB2 that the 6DX8 triode sections should be used in gain stages closest to the output, to avoid signal leakage from the output stage into the preamp.
( Although my hobby is building very low-powered guitar amps, parts prices in Australia make it very difficult to hit the $100 mark. 🙁 )
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I had the concertina and output connected to the same B+ node. After isolating the Concertina anode with a 1K and 2.2uF the oscillation seems to have gone away. More testing is necessary.
Hey Anatoliy, I get 144MHZ and something around 1MHz as well but this is much lower. I suspectecd power supply feedback.
This oscillation varies from 64KHz to 120KHz. It starts on the falling edge of a driving waveform, and grows as gain is increased until it is of constant magnitude.
This oscillation varies from 64KHz to 120KHz. It starts on the falling edge of a driving waveform, and grows as gain is increased until it is of constant magnitude.
Just wondering how the winner will be judged in this challenge.
I am a Luthier of over 30yrs & it seems this is going to hard to judge.
The only real way is by listening to audio clips or by running simulations on the final circuits to check for harmonic content, distortion etc.
If for instance the winner is chosen by circuit configuration or technically correct circuits then more than likely the sound will be ordinary & more suited to Audio rather than guitar.
Many technically "incorrect" circuits sound far better than technically correct circuits when it comes to guitar amps.
A lot of technically correct circuits lack pleasant harmonic content.
Maybe there should be minimum values set of say second harmonic content etc.
One of the worst designed tube amps turned out to be one of the best--the Fender Bassman.
Some insight to how this will be judged would be helpful & maybe minimum values of harmonic content should be set.
No one wants a clean signal for a guitar amp, not even a small practice amp.
I will give this a go myself but how is it going to be judged, with all due respect, I don't think the technically correct Audiophile can judge this????
Cheers
I am a Luthier of over 30yrs & it seems this is going to hard to judge.
The only real way is by listening to audio clips or by running simulations on the final circuits to check for harmonic content, distortion etc.
If for instance the winner is chosen by circuit configuration or technically correct circuits then more than likely the sound will be ordinary & more suited to Audio rather than guitar.
Many technically "incorrect" circuits sound far better than technically correct circuits when it comes to guitar amps.
A lot of technically correct circuits lack pleasant harmonic content.
Maybe there should be minimum values set of say second harmonic content etc.
One of the worst designed tube amps turned out to be one of the best--the Fender Bassman.
Some insight to how this will be judged would be helpful & maybe minimum values of harmonic content should be set.
No one wants a clean signal for a guitar amp, not even a small practice amp.
I will give this a go myself but how is it going to be judged, with all due respect, I don't think the technically correct Audiophile can judge this????
Cheers
I don't see why you call the Fender Bassman "worst designed" it is not very different than the black face Fenders.
I'd guess that the reason it sounded so good was that it had a bigger output
transformer.
They say it was not a very good bass amp, but an excellent lead amp.
Well, yesterday in the flea market i picked up an ultron el84, 2*Ei PCL86 Tungsram ECL86 for 4 euros (all NOS). I also have a nice polam ECC83 for 2 euros (same source).
As I said, the big tip: custom made cheap iron from local winders in Greece! Only reason I'd consider going to a GU50 is the cheap enough iron. And the coolness factor!
It was a terrible design because it was designed for bass guitar.
Hey costis_n, I'm not sure if our efforts are going to be voted or not. I have a feeling though that they're going to be killer sounding amps. Butchering lesser gods.
Speaker design was not very advanced in the late 1950s, so you blame it on the design?
The main limitation as a bass amp was the speaker. It was available as a head amp only
so the speaker was not really a limitation in other uses.
There is nothing technically "wrong" with the Bassman amp design as I see it and it was in
fact the basis (nearly an exact copy) for the early Marshall amplifiers. Many features are in
fact copied to this day, diff amp splitter, tone stack, etc. by leading names in the business.
Why is the above a quote?
By the late 30s speaker design was fairly advanced. Look at Paul Klipsch K-Horn, I think he had it designed before WWII.
Guitar amp speakers were probably cost driven and therefore simple single open enclosures.
Guitar amp speakers were probably cost driven and therefore simple single open enclosures.
Rice & Kellogg not only are a breakfast cereal recipe, but are also credited with the modern dynamic loudspeaker ca. 1924. The first sealed box system was about 1958. So give Leo Fender a bit of credit. There was no internet, few publications so info traveled slooow.
Fender Bassman reminds me a funny story, as if after WW-II one factory got an order to make a civil production. They started manufacturing coffee grinders, but still produced AK-47 machine-guns...
Similarly, when Hi-fi companies tried to make lead guitar amps they made something that disappeared very quickly.
Similarly, when Hi-fi companies tried to make lead guitar amps they made something that disappeared very quickly.
The moral of this story is: save pennies here, save pennies there...voilá....a great guitar amplifier has been made.
The Champ, The Bassman, maybe another great one coming soon? The 100$ challenge. 😀
The Champ, The Bassman, maybe another great one coming soon? The 100$ challenge. 😀
I have my Super-Mini Champ on breadboard already. It has a switch: Under-Champ and Over-Champ. Under-Champ is cleaner with less sag (triode mode), Over-Champ is dirtier with more sag (56K in screen grid from doubled B+) than original Champ. However, I had to use 12" speaker to get right sound, but tubes are still sub-miniature.
Micro-Bassman will use 6BA6 output tubes.
Vented speaker design, indeed most speaker design was cut and try in those days, that is a fact. While the sealed box speaker was already invented and patented by Olson, the Acoustic suspension variant was new and innovative in those days. We have come a very long way in loudspeaker theory and design.
I think I have tamed the oscillation, unless all three gain controls are cranked to 11.
I can't find the article right now but I used a step filter on the third gain stage. I can reduce the overall gain slightly and eliminate it completely.
Unfortunately it also rolls off some high frequencies.
I can't find the article right now but I used a step filter on the third gain stage. I can reduce the overall gain slightly and eliminate it completely.
Unfortunately it also rolls off some high frequencies.