Pin voltages for 100 watt Plexi &nbs

[Geezer]

  Hoffman Amplifiers
    > Tube Amp Building - Tweaks - Repairs
        > Pin voltages for 100 watt Plexi      
 
<< Prev Topic | Next Topic >>  
Author  Comment  
Jim SS
Junior tube assistant
Posts: 6
(2/15/04 12:31 pm)
 Pin voltages for 100 watt Plexi
--------------------------------------------------------------------------------
 I finished my Hoffman 100 w plexi and she sounds great. I am just curious as to what are the range of voltages I should be reading (tube pins/B ). I can't find these any where in the library of information.
Any help is appreciated.
BTW, thanks Doug for having a forum like this, it has provided me with countless hours of enjoyment in reading and building.

Thanks - Jim
 
GroundhogKen
Forum Moderator
Posts: 2267
(2/15/04 6:33 pm)
 Re: Pin voltages for 100 watt Plexi
--------------------------------------------------------------------------------
 My brother's '73 1959 Super Lead has 450v on the EL34 plates. I can't remember the other voltages.

The Marshall voltage chart reprinted in Pittman's book shows the following voltages:

Power tubes: 453v plates

PI tube 1,2,3,6,7,8
210, 0, 18.5, 195, 0, 18.5

Middle tube 1,2,3,6,7,8
153, 0, 1.1, 260, 153, 153

First tube 1,2,3,6,7,8
135, 0, 1.0, 195, 0, 1.5

These numbers were probably +/- 10% at least.

Ken
 
Jim SS
Junior tube assistant
Posts: 7
(2/16/04 9:56 pm)
 Re: Pin voltages for 100 watt Plexi
--------------------------------------------------------------------------------
 Thanks for the reply.
First, the EL34 plates have only ~230 volts. The preamp tubes are a little lower also.
Now, the amps sounds great, the bias is correct, and the amp works fine. is this a problem.

Thanks Jim
 
GroundhogKen
Forum Moderator
Posts: 2274
(2/16/04 10:18 pm)
 Re: Pin voltages for 100 watt Plexi
--------------------------------------------------------------------------------
 It looks like a problem.

Did you use this power circuit: ???
www.hoffmanamps.com/images/M100.gif

I'm suspicious that you grounded the center tap of your power transformer secondary. If you did, two out of 4 of the diodes in the rectifier bridge are now history. The center tap should be connected to the junction of two series 100uF caps where each 100uF cap is actually two 50uF caps paralleled.

It's biased correctly? That means that with 230v on the EL34s you need about 75mA flowing in each tube. Did you measure this?

Sounds Great??? It might but I doubt it has much power. A 100 watt Plexi should make you windows rattle with real authority.

Ken
 
Jim SS
Junior tube assistant
Posts: 8
(2/16/04 10:34 pm)
 Re: Pin voltages for 100 watt Plexi
--------------------------------------------------------------------------------
 It looks like a problem.

-- I agree!s

Did you use this power circuit: ???
www.hoffmanamps.com/images/M100.gif

1. yes

I'm suspicious that you grounded the center tap of your power transformer secondary. If you did, two out of 4 of the diodes in the rectifier bridge are now history. The center tap should be connected to the junction of two series 100uF caps where each 100uF cap is actually two 50uF caps paralleled.

It's biased correctly? That means that with 230v on the EL34s you need about 75mA flowing in each tube. Did you measure this?

2. yes, with a bias meter between the tubes and the tube sockets

Sounds Great??? It might but I doubt it has much power. A 100 watt Plexi should make you windows rattle with real authority.

3. Yes, it sounds great, I have a 100w plexi reissue wired by Gerald Webber to compare it to. My build is not quit as loud, but certanly loud enough for my neighbors to be able to hear the thump in their house (I live in a 4500 sq ft house and I'm playing in a finished basement).

Thanks for your help Ken


 
Jim SS
Junior tube assistant
Posts: 9
(2/21/04 11:24 pm)
 Re: Pin voltages for 100 watt Plexi
--------------------------------------------------------------------------------
 Ken,
Thanks for all your help. You were right, I did not notice but I had grounded the center tap of my power transformer secondary.
Voltages are now correct, and the amp is MUCH louder!

Again, I can not tell you how helpful this forum has been for me. It has been a long time since my college physics courses (that I did not pay as much attention to as I should have).

-Jim
 
 
 << Prev Topic | Next Topic >>

 

 Topic Commands
 Click to receive email notification of replies
  
jump to:  Tube Amp Building - Tweaks - Repairs (New)New Product Suggestions (New)Praises - Complaints - Suggestions (New)Buy - Sell- Trade (New)Archives (New)    
 

- Hoffman Amplifiers - Tube Amp Building - Tweaks - Repairs - Hoffman Amplifiers -



Powered By ezboard® Ver. 7.32
Copyright ©1999-2005 ezboard, Inc.