Hoffman Amplifiers
> Tube Amp Building - Tweaks - Repairs
> Bassman 6L6 to EL34 conversion...
<< Prev Topic | Next Topic >>
Author Comment
mtotheb
Junior tube assistant
Posts: 4
(2/18/04 11:50 am)
Bassman 6L6 to EL34 conversion...
--------------------------------------------------------------------------------
It's a '67 silverface "bassman amp" head, but it's in the very early stages of the silverface era so it's essentiallya blackface. First off, can the power tranny handle the additional filament draw of the EL34's and secondly does anyone have a link or a description of the proper mod to do this.
The ones I've found are the one on schematic heaven:
www1.korksoft.com/~schem/...l34mod.htm
And this is from the London Power site (
www.londonpower.com) with respect to installing EL34's in a Twin:
"Keep in mind that you can only install one pair of EL34s (with no other power tubes plugged in) because of the limitations of heater current supply, so you only have to modify two tube sockets. You should add in the spring-type tube retainers, available from New Sensor. Internally, you have to remove the end of the 1k5 grid stopper resistor and the hook-up wire (from pin-1). Connect the resistor and the wire together and stand the resistor up, clear of any other connections. Tie pin-1 of each modified socket directly to the raw bias supply - this is the junction of the 3k3, 8 uF, and tap on the bias pot. "
It seems that both have different approaches as to where to connect pin 1. The schematic heaven mod says to connect it to pin 8 (ground), while the London Power mod involves tieing it to the raw bias supply? Can anyone clarify this mod?
Thanks!
gammelnalle
Junior tube assistant
Posts: 10
(2/18/04 3:47 pm)
Re: Bassman 6L6 to EL34 conversion...
--------------------------------------------------------------------------------
Kevin O'connor states by doing this you'll save the tubes much better. He seems to be very supect to EL34. You should get his books TUT 1 and 3 . they are execellent books if you wanna build or tweak in tube circuits.
I THINK!!! it was Traynor who did this modification... If you want to I can check it out
ganzonimx
Hey get your own solder
Posts: 392
(2/18/04 6:27 pm)
Re: Bassman 6L6 to EL34 conversion...
--------------------------------------------------------------------------------
I would follow the instructions from Schematic Heaven to do the conversion.
You will have to disconnect the grid resistor from pin 1 and connect pin 1 and 8. That's because the El34 has no internal connection of the cathode and the control grid as it is the case in the 6L6. Look up a tube data sheet to check on this detail. In a 6L6 pin 1 has no internal connection so it can be used to solder on something.
A EL34 needs 1.5A for there heaters, the 6L6 only 0.9A. I would assume that the stock PT would not handle this additional required 1.2A. You would have to change the PT. You could use the TBPT Doug sells, it has a 6A heater winding, just tap off the 5V winding as you will not use it.
Cris
Wanabejimi
Junior tube assistant
Posts: 50
(2/18/04 7:36 pm)
Heaters
--------------------------------------------------------------------------------
You could get a 6.3 v filament transformer. They show up pretty regularly on ebay. Gerald Weber advocates using the filament transformer for the power tubes and using the stock PT to heat the preamp.
ganzonimx
Hey get your own solder
Posts: 394
(2/18/04 8:20 pm)
Re: Heaters
--------------------------------------------------------------------------------
Yes, that's definitely the cheaper way...
Cris
Lucid Alice
Senior tube assistant
Posts: 161
(2/18/04 10:41 pm)
Re: Heaters
--------------------------------------------------------------------------------
I haven't read Kevin O'connors books, but it is my understanding that he feels the EL34 and EL84 are the most unreliable power tubes ever made. (Funny, I've never found anyone who agrees. No doubt a lot of his stuff is really great. Like anyone else he has a few quirks that should be ignored.)
I have heard of the bias voltage to pin 1 thing before. Ampeg used to do that. In theory it is supposed to extend the life of the tube. That feature isn't necessary, it requires a mod to the circuit, and it can only be used with the EL34. That means you would have to mod your circuit again to change to another type of power tube. I would stick to the standard wiring... connect pin 1 to pin 8. This wiring scheme can be used with 6V6, 6L6, 6550, KT66, EL34, and there may be others.
Tiny Daddy
I will work on all amps
Posts: 495
(2/19/04 4:31 pm)
Tubes with external metal parts
--------------------------------------------------------------------------------
Tubes with external metal parts such as 6L6M (metal) or 6550 (metal base ring) connect the metal to Pin 1. If your build uses Pin 1 as a splice point this is bad, especially if it is the screen or plate voltage. I would ground Pin 1 because there is no way to know what tube someone may stuff in just to see what happens.
bluetonene
Junior tube assistant
Posts: 26
(2/19/04 5:26 pm)
Re: Tubes with external metal parts
--------------------------------------------------------------------------------
In my 5F6A's I routinly use 5 watt 1k resistors from pin 6 to pin 4. I also link pin 1 to pin 8 then to ground. I use Kendrick's PT's so the extra current is no problem. With this set up you can use 6L6, EL34, EL37 etc with just a bias adjustment. By using a 5Y3 rectififer the plte voltage will drop low enough to use 6V6's.
Doug or Ken,
I would use Dougs PT if they are interleave wound and spec out the same. Let me know.
Tom
<< 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.
Topic: Bassman 6L6 to EL34 conversion... (Read 6816 times)