Installing screen resistors in Traynor BassMate?

[jdw3]

Ok, I have modded my BassMate for a little more gain and mids. It sounds killer. It sounds the best just about dimed. That's where the trouble starts...

Into my 3rd set of el-84s and 6 fuses, I've discovered the need for screen resistors. From what I have read, the minimum for el84 tubes is 2.2K ohms at 5 watts.

Can anyone possibly shed a little more light on this? I have read about grid stoppers, do I need them?

[Phil_S]

Egad! 5W screen stoppers? Are you figuring that or did you read it somewhere? There is virtually no way you need a 5W stopper.

Egad again. Is this your amp? http://harmoniccycle.com/hc/images/guit ... YBA-2B.gif
Over 400V on the plates and screens. It is no wonder it eats tubes.

According to this schematic, you are dropping 2V across a 2.2K resistor. Let's call it 4V to be conservative. That's 2mA of current. (Ohm's Law, V = I * R.) 2mA * 430V < 1W. You can probably get by with a 1/2W resistor, but don't. Use the derated 1W.

[jdw3]

Egad! 5W screen stoppers? Are you figuring that or did you read it somewhere? There is virtually no way you need a 5W stopper.

Egad again. Is this your amp? http://harmoniccycle.com/hc/images/guit ... YBA-2B.gif
Over 400V on the plates and screens. It is no wonder it eats tubes.

According to this schematic, you are dropping 2V across a 2.2K resistor. Let's call it 4V to be conservative. That's 2mA of current. (Ohm's Law, V = I * R.) 2mA * 430V < 1W. You can probably get by with a 1/2W resistor, but don't. Use the derated 1W.

I read it in Kevin O'Connors book, TUT3.

That is almost the amp; mine has an additional cap in the bias section. BTW, I just discovered that cap is missing. It is a 8uf 250V. My print shows a 15K resistor between the negative ends of the 8uf and a 64uf cap. All I have in mine is the 64K. Is this my problem?

The 18K is there, as shown.

[xtian]

Check my thread on my recent YBA-2 build:

https://tubeamparchive.com/viewtopic.php?t=20310

Mine is stable.

[jdw3]

Check my thread on my recent YBA-2 build:

https://tubeamparchive.com/viewtopic.php?t=20310

Mine is stable.

I did see that. Very nice work. I have that amp as well, slightly modded for guitar. Everyone loves that sound! Mine is the 1x15 combo.

[M Fowler]

Yeah I've worked on this model amp and the plate voltage is just too high for today's tubes or maybe even NOS tubes.

310-0-310 PT if I remember correctly. Stick a zener diode on the center tap of PT reduce some voltage.

You sure do need to add screen resistors as well like your planning or already did. As far as I'm concerned 2k2/2w is better then no screen resistors any day.

Mark

[jdw3]

I'm sure I mentioned this in another thread, but I should add this here.

I added 2.2K/2W screen resistors to the BassMate. It fired up and so far no problems.

Lost a bit of volume, which is fine, and possibly a bit of gain. It sounds great though. Tubes are not pulsing with chords or anything.

Thanks for the comments!

[jdw3]

I'm sure I mentioned this in another thread, but I should add this here.

I added 2.2K/2W screen resistors to the BassMate. It fired up and so far no problems.

Lost a bit of volume, which is fine, and possibly a bit of gain. It sounds great though. Tubes are not pulsing with chords or anything.

Thanks for the comments!

[oldmacman]

I had one of these for years and ran it with Sovtek EL84M tubes and never blew it up. I would add the additional cap on the bias supply if it's missing -- certainly can't hurt.

They're great amps -- oversized OT and no global NFB makes it sound like the best Vox AC-15 you never played.

[Robertito]

I'm new at this, so maybe someone can tell me how to connect the screen grid resistors on the EL84s - I have the resistors, but I'm not sure where they go. I've installed them on 6L6s before, but not EL84s. Thanks!

[pompeiisneaks]

You can do it one of two ways.

1. install a terminal strip to the chassis and then connect the current wires to the grids there, and then jumper the screen resistors between that and the grid pins (g2 is pin 9 on the EL84, or if you can fit it, pin 1 isn't used on the EL84 so you could try to disconnect pin 9, connect it to pin 1 and jumper the screen resistors between pin 1 and 9. Not sure if there's enough room for that though.

Up to you.

~Phil

[sluckey]

...or if you can fit it, pin 1 isn't used on the EL84 so you could try to disconnect pin 9, connect it to pin 1 and jumper the screen resistors between pin 1 and 9.

I would not do that! Pin 1 is listed as internal connection. It's true, that many EL84s have nothing internally connected to pin 1, but some of the older tubes have pin 1 connected to pin 2 (control grid). That would be very bad!

Here's a pic of a RCA 6BQ5 that clearly shows pin 1 internally connected to pin 2.

[pompeiisneaks]

oh damn, I saw on a pinout that pin 6 was internally connected, so I figured pin 1 seemed to be the only one free. but that's obv. for newer tubes. Good to know.

Nevermind, then, I'd suggest using the first idea since that's guaranteed to be safe at that point!.

~Phil