Hot top plate

[ecir38]

I have been using a 300B version of the G*S*G for quite a while and have noticed the top plate generates allot of heat between the 300B's and 6SL7. I am considering disasembling to drill some holes around the tubes to help out. I am thinking a 13/64 hole will work out, this is between 1/8 and 1/4 inch.

While I have this apart I was thinking of maybe upgrading to some better caps. I haven't read anything here where anyone has played around with swapping caps to see if there were any improvements that can be achieved so I am just wondering if this may be worthwhile to look at.

From the list of caps below, which ones would you change and with what if any?

C1, C2 0.22uF
C3, C4 220uF
C5, C6 100uF
C7, C10 22uF
C8, C9 120uF
C11-C14 4700uF

[SDS-PAGE]

You might want to look at upgrading the coupling caps (0.22uF 630V). Hovland and Auricaps are pretty good. They run around $15 each. I don't know if changing any of electrolytic bypass caps will make any audible difference, but if you wanna upgrade those look at Blackgate caps. They are supposely ones that are used in Audio Note SET amps. Angela sell them.

Honestly, you might get better results experimenting with different 300B tubes or even 6SL7 tube, rather than changing caps.

How hot does the top plate get? I had a ST-35, whose top plate was hot enough to burn my fingers.

[ecir38]

I need to put my infrared temp gun to see the temparture range. It is to hot to touch for more than a few seconds. Just fiqured if I could could get the temp down that it would only help. It's just my feeling that if you could keep the temp down on any electronic or mechanical equipment that it usually makes it more effecient. Not saying there will be audible differance just more effeciant.

I kind of figured that C! and C2 would be the ones to play with. Don't know what brand yet since all of them have there differant characteristics.

I played with the 6SL7 and like the Slyvania VT-229. A freind of mine has Sophia mesh plates, WE's and TJ's that he said I can play with just haven't got to it yet. Unfortunatly the 6AX5 doesn't offer much to swap with, using a GE right now, since rolling the rectifier in some of my other stuff has proved to make more of a differance than everything else.

[SDS-PAGE]

I see. As far as the rectifier goes, you will get more output power using the SS rectifier. The 6AX5 drops around 60V (?) and 300Bs will be more at home without it, and so will your ears. -Min

[hembrook]

I think that 300Bs do need more voltage to get best sound, 275V plate to cathode is not that much for a 300B, but didn't you frag some caps and resistors when you went solid state? Just an FYI.

[Shannon Parks]

I think that 300Bs do need more voltage to get best sound, 275V plate to cathode is not that much for a 300B, but didn't you frag some caps and resistors when you went solid state? Just an FYI.

Robert, the circuit can easily handle SS voltages as I believe the problem with Lee's first SS try was a runaway power tube that internally shorted with the aggressive bias due to the original cathode resistors. Those would need to be changed to around a 1200 ohm network, instead of 900 ohms. Note: I've never biased up the GSG with SS so I'm not sure what the optimal bias resistors should be.

[EWBrown]

I's SWAG that the hot plate is being caused mostly by the WW cathode resistors, especially if any are mounted on the upper surface of the board.

Some radiant heat can be picked up from the 300Bs, but the WW cathode resistors are the major source of heating.

Rather than upgrading / replacing the 300B cathode bypass caps, just add an extra 2.2 to 4.7 uF, 250v MKP (like Solen Fast Cap or similar) cap, this will help the higher freq response and effectively reduce any ESR and ESL that the electolytics may have.

/ed B in NH

[ecir38]

Thanks, after looking further into this, you are right about the heat is coming from above the cathode resistors area.

For the caps recommendation, are you saying bypass the bypass caps? This soulds like a good idea.

[EWBrown]

Yes, I've done that (bypass the bypass caps), in several amps, I used 2.2 uF / 250V MKP or MKC type caps, I had a bunch of relatively cheap ones that worked quite well, and they made quite a noticeable difference in most cases. The easiest approach for the PCB is to connect across each set of the cathode resistors, just leave enough lead length to allow some physical separation (just let tham hang down), so as not to get overheated by the hot resistors.

Just refer to the echematic here, I built up a very similar EL34 SE / UL amp, which is the first one I tried bypassing the bypass caps. He does it a bit more elaborately, I just used the 2.2 uF caps, across the electrolytics.

http://www.bonavolta.ch/hobby/en/audio/el34_4.htm

He did it for both the VA and the power staes, I generally just bypass the PA cathode cap.

/ed B in NH