Komet => 60 watts with 2*EL34 ?

[MKflo84]

Hello there,

I'm new here, I'm 21 and I'm a Mark Knopfler fan since my birth !
I recently began to build amps as I'm studying electronics and information science. My last "baby" is a SLO100 like amp and I want to increase my knowledge of electronics applied to tubes.

I've seen the Komet pics on this forum and I aslo tried to make some basic SPICE simulation of this circuit (especially concerning the frequency response).

I'm wondering how the Komet could be rated at 60W as I've read that EL34 are rated at 20W max. Does it mean that the tubes are "overvolted" in the Komet ?

I've used the SLO100 Power supply, feeding the plates at 500V, the B+2 (for the PI) at 390V, the B+3 at 370 (for the 3 12AX7)with bias set to -40V. The simulation gave me something like 115 mA through the OT (which is I think pretty high ?).

First, are the 3 preamp tube fed at the same B+ ? When I look at the pics I think that there might be 3 different B+.
I've seen some people here telling that the Komet Power supply is a pretty good design, could someone explain how it works ?
Here are the thing I guess from the pics :
TRANS (350VAC ?)=> GZ34=>two 80uF series with two 220K paralel (B+1) => choke =>two 80uF series with two 220K paralel (B+2) => R (value ?) B+3=>20u/500V => ??
There's two additional 20u that I can't identify in the PSU design.

For the bias, I guess that it's a classi design with diode / two 5u(250V)/10K 47K and 50K pot.

Any help will be much appreciated,
Florian

[ODwan]

First, are the 3 preamp tube fed at the same B+ ? When I look at the pics I think that there might be 3 different B+.
I've seen some people here telling that the Komet Power supply is a pretty good design, could someone explain how it works ?
Here are the thing I guess from the pics :
TRANS (350VAC ?)=> GZ34=>two 80uF series with two 220K paralel (B+1) => choke =>two 80uF series with two 220K paralel (B+2) => R (value ?) B+3=>20u/500V => ??
There's two additional 20u that I can't identify in the PSU design.

Any help will be much appreciated,
Florian

Hello, Florian!
I think the Komet uses the same B+ distribution as the Express. After looking at the photos again I think the PS layout would be:
TRANS (350VAC ?)=> GZ34=>two 80uF series with two 220K paralel (B+1) => choke =>two 80uF series with two 220K paralel (B+2) => R (value ?) B+3=>20u/500V =>diode => R (value?) B+4 =>20u/450V => R (value?) B+5 => 20u/450V
The diode is said to be in there to prevent the poweramp stealing current from the preamp. This would make the preamp stiffer I think.

Where do you hail from? Your name sounds a bit german to me.

Greetings, Timo

[MKflo84]

Hello again,
Thank you very much for your answer. I've heard som things concerning a "diode trick" in the power section. Now I can figure a little bit better what this trick is.

I know that my name sound German but in fact I'm french (and I speak a little german ). I'm a student at Supélec, which is actualy what we call a "grande école" pecialised in electronics and information technology, near Paris. I play guitar too, so that's why I'm very interested in tubes.

I guess that for the diode a 1N4007 should work fine.
I've some questions left :
-How do you think are the B+ plugged ? first tube and second tube+5, third +4, phase inverter +3, screens+2, plates +1?
-Any thought concerning the R values ?
-DO you think that a 60 mA per tube is right ?
Thanks
Florian

[MarkB]

I don't know about that amp, but you can boost the output of EL34s by raising the plate voltage and keeping the screen voltage low. EL34s are rated for 800V on the plates, but are rarely run that high in guitar amps.

[loverocker]

And the Marshall 50Wer puts out way more than 50W from a pair of EL34s. 90W at 10% THD, if I remember right. So maybe it's only a perfectly clean 50W that's hard to get?

And the number of people after a Wreck that's clean at 60W must be pretty low, right?

[ODwan]

To get more than 45-50W out of 2 EL34s the amp has to be biased somewhat into Class B. A certain high B+ voltage is also required.

I know that my name sound German but in fact I'm french (and I speak a little german ). I'm a student at Supélec, which is actualy what we call a "grande école" pecialised in electronics and information technology, near Paris. I play guitar too, so that's why I'm very interested in tubes.

I guess that for the diode a 1N4007 should work fine.
I've some questions left :
-How do you think are the B+ plugged ? first tube and second tube+5, third +4, phase inverter +3, screens+2, plates +1?
-Any thought concerning the R values ?
-DO you think that a 60 mA per tube is right ?

I am german. I had nothing to do with electronics till I got the idea of building a tube amp myself. So I started searching the web and now three years later I'm pretty well educated about tube circuits. I also dabbled in stompboxes some times. There are several sites with great info on tube amp circuits and how they work. I don't know hows your knowledge but I can recommend the AX84 site and Randall Aiken's site (http://www.aikenamps.com. I also bought Kevin O'Connors books which are really great.

You are right about the B+ connection. I don't know about the R's but you could try to read the colorcodes from the photos.
Do you mean the bias current of the powertubes or the preamp tubes?
For athe preamp circuit look at Express Var.2 in the Files section of this forum!

So long, Timo

[ODwan]

And the Marshall 50Wer puts out way more than 50W from a pair of EL34s. 90W at 10% THD, if I remember right. So maybe it's only a perfectly clean 50W that's hard to get?

And the number of people after a Wreck that's clean at 60W must be pretty low, right?

Normally output power should be measured at max clean output with a 1kHz sine wave.
The Komet site and KF talk about "true british watts" though I don't really know what that means.
Loverocker, any thoughts?

Timo

[loverocker]

From the original 50W test results (in Doyle's book): "In excess of 50W RMS typical power at clipping ...Typical output power at 10% distortion, 90W into 16 ohms"

So 60W sounds possible, although I reckon that in reality, there'd be little perceived increase in volume between a 50W and 60W amp. It's not like the difference between 10W and 20W