Peavey Delta Blues bias adjustment

[Leif H]

First post here, second mod to do, of many.

It looks like I've got to replace the output tubes in my Delta Blues.

As much as I'd like a auto biasing system that Traynor has, I'll settle for putting in a pot instead of the resistor.

I just want to be sure that Im on the right track, or tracks on the PC board hat is. Looking at the schematic, I'm confident that it's R65, but want to make sure.


Schematic.

And as the resistor's value is 33k, going with a 50k pot should be ok. will I need to go more than that?

Thanks

Leif

[Enzo]

OK, I give up, I don't see R65 anywhere. Where is it?

Did you mean R64?

R63,R64 form the voltage divider off the -36v supply for bias. Making R64 variable will make the bias voltage variable, yes. I am going to take a wild guess that you don;t plan to set the bias any cooler (higher bias voltage) so and value at R64 higher than the stock 33k would be pointless. But I guess 50k isn;t THAT much larger.

I prefer to install a pot with a series resistor. That way if the pot is turned to zero, the resistor is still there to ensure a minimum of voltage. SO if I added a 15k resistor to the pot, I could use a 25k pot. That makes 40k, pretty close to 33k. And now you can't accidentally turn the bias off. Plus now a lot more of the pot travel will be useful. If you only used a 50k pot, then half hte pot travel wou0ld be off limitws at least. With my version, you would have use of a lot more of the pot. That makes setting it less touchy as well as safer.

[Leif H]

Ever have one of those days...?

Enzo, yes R64, my aim stinks, but then again, the 4 and 5 keys, are awfully close together.

Thanks, I like the idea of the smaller range pot. and not wanting to fry them by being ham-fisted when tweaking things.

My next question is; Will the Test point for bias react instantaneously with the adjustment, or does it ramp up / down? as the plates heat / cool?

Cheers,

Leif

[Enzo]

If it lags any, it is not enough to notice. Let your amp warm uop for 15-20 minutes so the tubes all settle in. THEN set the thing.

Are you unhappy with the amp as it performs stock?

[Leif H]

If it lags any, it is not enough to notice. Let your amp warm uop for 15-20 minutes so the tubes all settle in. THEN set the thing.

Are you unhappy with the amp as it performs stock?

Love the amp, It's second hand, and the owner warned me that I will need to replace the output tubes, sometime.

[Enzo]

My point was that even with fresh tubes then, does the amp perform well as stock? In other words is this modification necessary?

[Leif H]

My point was that even with fresh tubes then, does the amp perform well as stock? In other words is this modification necessary?

It does, I'm being told that to replace the tubes, I've got to re-bias.

I was going to go again with the same EL-84.

Or, am I being handed a line?

The guy at the store seemed a little knowledgable, so I took his info at face value.

I know that Traynor's have an auto biasing featue, but, is that for switching output tube types?

Hmmm....

[Enzo]

I am not familiar with what Traynor is doing. Self biasing often just refers to cathode bias.

You are not being fed a line, but You are running into a philosophy. Your guy thinks all amps MUST MUST MUST be rebiased every time tubes are changed. Well the tire pressure in your car MUST MUST MUST be checked every time you get gasoline. They tell you that, but do you do it? And yet your car drives safely down the road year after year.

That amp left the factory as non-adjustable bias. SO did thousands of amps just like it. None of them are adjustable. The PV philosophy is to design amps that work fine without having to be adjusted. DO guys decide to modify their amps? Hot rod them a bit? Sure, and to them the amps may well sound better. But the point is the amp sounds great to you now. It is not adjustable now. The factory stuck tubes in it when they built it - there was no adjusting done then. There is absolutely nothing special about the tubes, they don;t pre-measure them or sort them.

To some folks bias is almost a religious term. I'd say it is way over emphasized. There is bias way too cold, and there is bias way to hot (the tubes get red hot), but in between there is a wide range of bias currents that are perfectly fine. It is not a critical adjustment. SOme guys live by the 70% rule - setting the bias current for tube pplate dissipation of 70% of max. Others think 80% is better, and some would rather see 60%, and yet these are all OK bias levels. No one is "wrong."

WHen you make an amp adjustable, 99.9% of the time it is to run the tubes hotter. No one ever wants to run cooler. This is all well and good, but the hotter the tubes run the shorter their lives will be. If you run this amp up to 70% it won;t be the difference of 6 weeks versus two years, I can't say how long your tubes wil last. But overall, the hotter they run the less time they'l do it. Many people find their tone Nirvana there and more power to them, the tube life is whatever it is. This is also in the PV mind when they design amps. They want good tube life.

SO I am not suggesting you shouldn't do this. Just want you to know what the deal is.

[Leif H]

Thanks for the reply...

Whether I'm being fed a line, it's hard to know, but thanks all the same, I'm going to re-tube, and leave the bias alone.

Just did some re-checking... I read this this amp is pretty much the same as the classic 50... but is, run more conservatively.

And now for the pre-amp out...

[Enzo]

The Delta Blues is identical to the Classic 30, not Classic 50, though they are all related.

The Delta BLues differs in the slightly larger cab and the speaker selection The Classic 30 has a single 12" while the Delta Blues comes in a single 15" or a 2 x 10"

Electrically the ONLY difference between Delta Blues and Classic 30 is the DB has a tremolo.

[Fabão]

Well, I got to say that Delta Blues is the best amp I've ever had.
But, since I changed my tubes, I've listening a cooler tone.
I got the JJ's now and they don't sound better than the tubes it had when I got it second hand.
Thing is, I'm no an expert in electronics so...I youtubed some videos and boooommmm...the bias adjustment was the solution.
So, here I am with my amp all striped and I can't find a bias adjustment pot.
I got to say that the non-adjustable bias is NOT working as good as it should.
Having the bias adjustable is a great thing 'coz you can adjust the tone as you like.
I'm not happier with the cooler I got now and I can tell that JJ's are great tubes.
It's missing the bias adjustment to be THE perfect tube amp.

[Fabão]

Well, I got to say that Delta Blues is the best amp I've ever had.
But, since I changed my tubes, I've listening a cooler tone.
I got the JJ's now and they don't sound better than the tubes it had when I got it second hand.
Thing is, I'm no an expert in electronics so...I youtubed some videos and boooommmm...the bias adjustment was the solution.
So, here I am with my amp all striped and I can't find a bias adjustment pot.
I got to say that the non-adjustable bias is NOT working as good as it should.
Having the bias adjustable is a great thing 'coz you can adjust the tone as you like.
I'm not happier with the cooler tone I got now and I can tell that JJ's are great tubes.
It's missing the bias adjustment to be THE perfect tube amp.

[Jazz P Bass]

Well, I got to say that Delta Blues is the best amp I've ever had.
But, since I changed my tubes, I've listening a cooler tone.
I got the JJ's now and they don't sound better than the tubes it had when I got it second hand.
Thing is, I'm no an expert in electronics so...I youtubed some videos and boooommmm...the bias adjustment was the solution.
So, here I am with my amp all striped and I can't find a bias adjustment pot.
I got to say that the non-adjustable bias is NOT working as good as it should.
Having the bias adjustable is a great thing 'coz you can adjust the tone as you like.
I'm not happier with the cooler I got now and I can tell that JJ's are great tubes.
It's missing the bias adjustment to be THE perfect tube amp.

How do you know that the JJ's are biasing up 'cooler'?

Has the bias current through the tubes been measured?

[DRH1958]

The only way to be sure of exact proper bias is to measure current through the tubes. Voltage is not an indication of proper bias, current is. -14 volts may be perfect for some tubes and not so for others. But as Enzo said, they have been making non-adjustable bias amps for years. Peavey must think it's design doesn't need to be this exact. -14v should be reasonable for most tubes.
If you like your tone better with the old tubes, put them back in if you like the tone and it works like you want. Another option is to hunt down the same tubes that you like the tone of. You don't have to buy new JJs, you can find older tubes that are the same as the ones you like.
And the person who said you need to re-bias every time you change output tubes is just quoting things without knowing what is happening with this amp. You can't rebias this amp because it has no adjustment. Yes, generally, you want optimum bias when replacing tubes, but you have to evaluate if it is worth it to modify the amp to make it adjustable.

[Enzo]

And just a note, I think in general, the JJ tubes are a little less bright than the Sovteks.