Fender Blues Junior IV Tube replacement and Bias Question

[Jam]

I got a Fender Blues Junior IV amp and I was gonna change tubes to a different brand but same part number so do I still need to bias it? Or does it have to be same identical brand and part number? I know it has a fixed resistor it and no trim pot? Thanks for any info

[Jon Snell]

The mk4 has the updated bias supply and runs a little cooler. If you are UK based, don't forget to change the primary tapping from 230 to 240volts!
Fit quality valves, NOT JJs or Chinesium, use Sovtec or Eastern Block valves, they are built to last.

[nickb]

Sorry - I have to disagree with the above. I have a pile of bad Sovtek EL84's that just can't handle running hot: grids get hot and they start to run away. I've have zero JJ's in that category. Generalization is tricky so YMMV..

[Jon Snell]

Sorry - I have to disagree with the above. I have a pile of bad Sovtek EL84's that just can't handle running hot: grids get hot and they start to run away. I've have zero JJ's in that category. Generalization is tricky so YMMV..

YMMV ?
Sovtec 6p14p-ev are military grade superb valves with a higher dissipation than EL84/6BQ5. I can only speak from experience not guess work.
Out of aprocimately 500 pairs of Sovtec manufactured 6p14p-ev valves I have only had one faulty and that is because pins 8 & 9 were connected together within the valve. the lead wires were touching, not causing an issue if the EL84 valve base was wired correctly as pin 8 is not used.
Look at the specification sheet for dissipation etc.

[pdf64]

It seems something of a misrepresentation to put NOS military valves, that have have had the Sovtek branding applied, as an example of a typical Sovtek product?
Recommending a specific product is one thing, the whole brand is another.
Regarding the published valve info, unless the rating system qualified, the published data isn’t really comparable. So apples and oranges. People already mungle up design centre and design max ratings, leading to the misapprehension that 6L6GC are 20% beefier than EL34.
http://www.one-electron.com/Archives...on%20Tubes.pdf

[Jon Snell]

Any valve from these manufacturers; MELZ, Polaron, Svetlana, Reflector and Ulyanov, lower NEVZ (Novosibirsk city), Gazotron (Rovno City, Ukrane),

exceed the required specification. As I mentioned, I can only speak from experience.
Also they are sold at the correct price, not inflated like some sellers do.

[nickb]

YMMV ?
Sovtec 6p14p-ev are military grade superb valves with a higher dissipation than EL84/6BQ5. I can only speak from experience not guess work.
Out of aprocimately 500 pairs of Sovtec manufactured 6p14p-ev valves I have only had one faulty and that is because pins 8 & 9 were connected together within the valve. the lead wires were touching, not causing an issue if the EL84 valve base was wired correctly as pin 8 is not used.
Look at the specification sheet for dissipation etc.

YMMV = Your mileage may vary. The point being that both brands and specs vary enormously so you have to be cautious about making blanket statements.

Guesswork? That's absurd. You have no grounds on which make such a assertion. My post is based on real experience too. I've bought tubes by the pallet load, I currently have a bit over £10K in stock, and I still spend thousands at time on them and I think qualifies as quite a bit of experience.

Focusing on Electro-Harmonix's Sovtek EL84, as a reasonable person would guess that is what you meant by Sovtec (sic). On the whole I've been pretty happy with them. But when you get a few that thermally run away it's natural to want to try something else so I switched to JJ's and haven't had a single failure. No doubt that will change one day.

I haven't really gone for 6n14p (or -ev). I had a rather poor experience when I did try some. What arrived was clearly used and some were actually magic eyes in the box. That said I didn't have any issues with them but it was a small sample, maybe 20? But it did spoil my trust. I prefer to deal with manufacturers I can get on the phone with. It was a long time ago. Prices seem to have gone up quite a bit since then.

[pdf64]

Any valve from these manufacturers; MELZ, Polaron, Svetlana, Reflector and Ulyanov, lower NEVZ (Novosibirsk city), Gazotron (Rovno City, Ukrane),

exceed the required specification. As I mentioned, I can only speak from experience.
Also they are sold at the correct price, not inflated like some sellers do.

Do you mean that you are able to buy directly from the manufacturers mentioned? If not, then what vendors can you recommend?

Regarding the 'any valve' thing, nickb has found issues with Russian made 12AX7 type valves, when used in typical DCCF guitar amp circuits, see https://music-electronics-forum.com/...47283-?t=46108

[Jon Snell]

I am sorry if I offended you in any way, that was not my intention.

How could I know if you are a qualified engineer, as I am or a novice.
There is a very wide spectrum of readers on this site from little experience Novice Dabblers to Btec skills with 50 years behind my belt, which is why I refered to guesswork.

[Jam]

That went over my head with all of that . LOL