Silvertone 1484

[wbfree]

I have a 1484 with a fixed bias and I wanted to change it to an adjustable bias or a cathode bias, I know it is possible but I wanted to know if it was expensive to have done, I am not knowledgeable enough to attempt this. I was more curious as to thoughts on how much, or little, a cathode bias would change tone.
Thx
Burnie

[10thTx]

There is an editable schematic here (if you have ExpressSCH program downloaded which is free):

http://www.el34world.com/Forum/yabb2/Ya ... 1163855375

The 1484 has the vibrato tied into fixed bias, so you would have to change that. It's possible to make the amp cathode biased. However, I think you will have to experiment with the vibrato section after doing so.

With respect,
10thtx

[wbfree]

Thx 10thTx
My amp is no longer stock and I believe that tube is now part of a gain stage. Let me be up front in saying I am a complete newcomer to this technology but it is fascinating. I don't know much at all but I want to learn. I don't like taking my amps to someone else and not being able to achieve what I want in terms of tone. On this particular amp I want to run el34's, I like them and the sound I get with them but the amp is not biased for them and I thought a cathode bias would be a great start. I will get that program.
Best,
Burnie

[CaseyJones]

I have a 1484 with a fixed bias and I wanted to change it to an adjustable bias or a cathode bias. On this particular amp I want to run el34's, I like them and the sound I get with them but the amp is not biased for them and I thought a cathode bias would be a great start. I will get that program.

Best advice I can give... don't do it.

The transformers on the 1484 are adequate for the stock circuit but they're on the weak side for EL34s. IMHO using EL34s in that amp will let the smoke out, amps won't work without smoke.

[wbfree]

Point taken, I am just an inveterate tweaker, but will take your advice-thank you.

[10thTx]

Out of curiousity, how would you describe the current tone you have and what do you want to be different? That question and answer might lead to other suggestions that might be more reasonable for the amp?

You can easily convert the reverb into using a Fender replacement R/T and reverb tank with one 12Ax7 ............ or parallel the V1 tube for 30% (approx) more gain.

You've got a bunch of tubes in that amp that will allow you numerous possibilities. You could change it into a Marshall type typology with a paralleled V1 and cathode follower using a 33k resistor for the mids of the tone stack & LTPI similar to this:

http://www.el34world.com/Forum/yabb2/Ya ... 1199629316

There is an entire library of editable schematics to choose from here:

http://www.el34world.com/Forum/yabb2/Ya ... Schematics

With respect, 10thTx

[Phil_S]

I don't seem to have what's needed to read the referenced schematic. I'm looking at this one:
http://www.schematicheaven.com/bargainb ... ne1484.pdf
The bias supply is set by R60, a 100K ohm resistor. You can change that to a 50K ohm pot and 68K resistor in series. That will give you adjustable fixed bias. This means you will be able to control the bias voltage within a limited range. This should be relatively easy to do, not expensive, and is IMO both harmless and desirable.

If you want to convert this amp to cathode bias, there is a lot more work and know-how involved because of the relationship between the tremolo circuit and the bias circuit. If you want an example of a cathode biased amp with tremolo, look at a Gibson Scout GA17-RVT (bias modulated) or Gibson Minuteman GA20-RVT (LDR modulated), both available at schematicheaven.com.

If it were my amp, I wouldn't bother converting it to cathode bias, but I would put in the 50K ohm pot. IMO, an amp tech with one year of experience can handle adjustable fixed bias mod.

You didn't say...what's your motivation for making a change? That might influence comments.

[dobbhill]

Phil: you mean 50k ohm 50 ohm pot, yes?
D

[Phil_S]

Phil: you mean 50k ohm 50 ohm pot, yes?
D

Yes. I'll edit the post.

[wbfree]

Phil: you mean 50k ohm 50 ohm pot, yes?
D

Yes. I'll edit the post.

Thanks guys for the help and info. I will try this
Burnie