Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Re: plexi reissue questions 1987x

9/18/1999 9:33 PM
Dave M.
Re: plexi reissue questions 1987x
Ted, what's the plate voltage? Currently I'm running my '73 1987 with 40ma. The amp runs 420 volts st the plates.  
And now, a word from our sponsors:

9/18/1999 10:05 PM
Ted Matsumura

At -45.0ma bias, I'm getting 473 plate VDC with Sylvanias, and with -40.0ma bias, I'm getting 463 VDC for a range of ~18-21 watts static dissipation.  
9/19/1999 12:40 PM

I second Bruce's comment. I have watched (with interest!) as my own screens have lit up on heavy overdrive. You can see them through the slots in the EL34 plates. I run mine hotter than most, using a single 470-ohm screen resistor. The original plexi's use no resistor at all, but of course had Mullards which could probably take the punishment a lot better! It is a good idea to run modern tubes each with their own 1K-5W screen resistor for reliability's sake. The only downside is a slight (probably imperceptible) loss of output power (as much as 15W worth roughly) going from no screen R to 1K's. I have not measured it, but you may also get a little more cross-over distortion with the screens running cooler but I can't say for sure.  
I have found in my own testing that if you're going to play heavy stuff for an hour or more at a shot, the power transformer can run pretty hot (as can the stock OT's) and I'd be careful of going too high on the bias. You can leave the back panel off for better ventilation, maybe put a fan on it too. The current draw of the EL34's clipping hard is quite high, as is the duty cycle. YMMV, these are all just things to be aware of.
9/19/1999 4:22 PM
Ted Matsumura

OK, thanks Joe and Mook,  
I think the flicker is OK, and I'll keep the Sylvanias in there at -40ma for awhile and put some mileage on the tubes and amp. First thing I did was take the back panel off, and a fan in back of the unit whenever played, but now the whole chassis is out, still with a fan in back. The PT and OT have never even gotten warm to the touch on me yet, but then I rarely have volumes cranked. Yes, the 5K 1W big resistors are part of the reissue design on each tube socket.
9/19/1999 12:42 PM

Yes, I have the same layout. The first resistor you mentioned (56K) has a "47K" written in white on the PCB, but the actual resistor is 56K.  
As for the tubes "winking" when you play...this to me is a regular occurence. My amps do this too.  
Sounds like your PCB is OK.  

<<First Page<PrevPage 2 of 2