About Blues jr's original trannies.

[Twosteps]

Hi guys, I have some questions about the bjr's trannies. I've got this old beaten up ver. 1 bjr from a friend who has given up on it. It had been played 5-7 nights a week for 7-8 years and now it won't work properly anymore. Somebody had tried to fix it but it still not working properly so I ask if I could have the corpse.

I'm not going to fix this one - too much hassles. I'll rebuild it into something either Hoffman's version bjr or a completely different amp.

Here are questions regarding the transformer
(I'm trying to stick with the orig. trannies if they're not broken to save cost here)

- First of all, how do I check if the old trannies still functioning or not?
- Assuming that they're still ok they seem to be a bit different from the trannies that I used to, does anybody have their wiring diagram? Can anybody explain to me something about these trannies?
- Do you think it's possible to build some British style el84 powered with these trannies.

Thanks in advance guys, cheers!

[sluckey]

The schematic is available in the schematics library at the bottom of this page. Get it.

To check the PT just plug into the wall and use a voltmeter to check all secondary voltages. Check OT by measuring resistance from primary CT to each plate lead. Probably a couple hundred ohms and both readings should be approx. the same. Secondary windings should measure very low (maybe even zero) ohms and should have no continuity to primary windings.

Do you think it's possible to build some British style el84 powered with these trannies.

Yes.

[Fresh_Start]

Matchless Lightning
Marshall 18 Watt (TMB variant)
Vox AC-30 top boost channel with 2 EL84 power amp of your choice

Or you could make an 18 Watt Stout TMB with reverb. "Extra" 12AX7/12DW7 for one tube reverb Tubenit's way.

Hoffman's tube board for this chassis looks great. There's nothing wrong with the transformers. Three 9-pin preamp tubes and a pair of EL-84s can go many different directions.

Cheers,
Chip

[Deric]

I did an S-2 "Jim and Leo" inspired Blues Jr. rebuild for a friend several years ago.  He LOVES it.  Marshall 18 watt on one channel 5E3 on the other (sort of...).  He's been gigging it 2-3 nights a week for 6+ years.  Still going strong.

[Twosteps]

Great! You guys have just confirmed that this is a great place to be  well, some more questions assuming that I'm going with the old trannies

- Both pt and ot are limited to the bjr specs, if I'm gonna go with any 18 watts style amp what do I have to adjust in the circuit to make it go with the under spec trannies? Or do I just drop them in and get the 15 watts version of those amp?

- I have to admit that the pt's secondary is unusual to me. Could somebody explain to me why is it like that? Is it because it has solid state rec. so it simply cut out 5v tab? Is the 20v tab to feed the solid state rectifier, right? It seems like I have to go with solid state rectifier here.

- Also the pt's secondary is fixed at 240v, most of those 18 watts are looking for ~290v more or less. What do I have to adapt here?

The only wiring diagrams that I could find are from Hammond. They say it's a drop in so should be pretty similar to the original. Here it is..

[eleventeen]

- Both pt and ot are limited to the bjr specs, if I'm gonna go with any 18 watts style amp what do I have to adjust in the circuit to make it go with the under spec trannies? Or do I just drop them in and get the 15 watts version of those amp?  <---Don't worry about this. -


I have to admit that the pt's secondary is unusual to me. Could somebody explain to me why is it like that? Is it because it has solid state rec. so it simply cut out 5v tab? <---correct.


Is the 20v tab to feed the solid state rectifier, right? <---no, it feeds the power supply for the op-amp preamp which looks like it wants a +/-15 volt supply. Very typical.  The 20 v winding has nothing to do with the "high voltage tube" power supply.


It seems like I have to go with solid state rectifier here. <--pretty much, unless you want to go buy a 5 volt transformer to power the filament for a 5Y3 or a GZ34. Not expensive, but the lack of a CT on the HV secondary means you are forced to go to a bridge (4-diode) rectifier anyway, so you can't use a 5Y3/GZ34 EVEN IF you buy the external fil transformer. Ergo, forget the tube rectifier.

Now you have to decide whether you will be building a fixed bias amp (where the main power supply is tapped or rectified to provide about -30 to -50 volts to the GRIDS of the 6BQ5 or 6V6 outputs, OR, if you are going to build a cathode-bias amp where the output tubes bias themselves. Fender blackface amps are typically fixed bias, a bit more complicated, but not much. Many other amps are cathode bias, self biasing, a tad simpler. There is no overwhelming consensus on which is better. I love blackface amps, but the appeal of cathode bias is the self-regulating aspect. No bias adjustment, even after tubes age or get replaced.

*IF* you decide upon fixed bias (which is a brain tweaker because the fixed bias voltage level *IS* variable, but the circuit is called what it is called) then the 20 volt winding could be fed to a voltage doubler to make your ~~-35 volt negative bias, OR, the main bridge rectifier can be tapped. Pick your circuit type and we can help you with where to go from there.

- Also the pt's secondary is fixed at 240v, most of those 18 watts are looking for ~290v more or less. What do I have to adapt here?  <<---Nothing, after filtering, the bridge rectifier should produce roughly 1.4 times the secondary AC volts, thus, 240 * 1.4 = 336 unloaded. In the circuit from Doug's library, the highest B+ is 329. You are good to go exactly as you are. Forget the tube rectifier, this transformer and your intention to reuse it essentially forces you to go 4-diode bridge, end of story.

[Twosteps]

Thanks eleventeen, thanks everybody! This is getting fun  I'm learning something now and even if this plan's not gonna work out for whatever reason this should still be a good lesson and a brain exercise for me!

Say the trannies are good and I'm gonna keep em also I'm gonna go with solid state rec. my top choice now probably would be a Carmen Ghia clone. No way this is gonna be an exact clone for many reasons cab construction, tubes, trannies, etc... but I like the idea of having just 2 knobs that really work plus I do prefer the hardwire route, with the limited space in the bjr chassis it seems like a reasonable way to go.

Here's the schem

[sluckey]

The only wiring diagrams that I could find are from Hammond.

Then you didn't look at the schematic in the tube schematics library at the bottom of this page.

*IF* you decide upon fixed bias (which is a brain tweaker because the fixed bias voltage level *IS* variable, but the circuit is called what it is called) then the 20 volt winding could be fed to a voltage doubler to make your ~~-35 volt negative bias, OR, the main bridge rectifier can be tapped. Pick your circuit type and we can help you with where to go from there.

The BJ IS a fixed bias amp and that 20VAC PT winding is used for the negative bias supply as well as the SS circuitry. A 6BQ5 (EL84) only needs -10VDC to -20VDC for proper biasing, so a simple half wave bias supply is sufficient.

It seems like I have to go with solid state rectifier here.

Not necessarily. You can use a hybrid bridge by using an EZ81 tube rectifier with two 1N4007 diodes. Several examples that do that right here on this forum or see this link...

     http://www.valvewizard.co.uk/bridge.html

Is the 20v tab to feed the solid state rectifier, right? <---no, it feeds the power supply for the op-amp preamp which looks like it wants a +/-15 volt supply. Very typical.  The 20 v winding has nothing to do with the "high voltage tube" power supply.

The 20VAC winding is also the source for the negative bias for the output tubes. If you build a cathode biased amp such as the Carmen Ghia, or Vox AC-15 lite, or Marshall 18, or Matchless Lightning, etc, etc, you will not need that 20V winding.

The November amp is another good candidate. It's a fixed bias power amp with a Plexi preamp. Sounds good if you like Marshall crunch.

[Twosteps]

Very clear sluckey, very clear explanation. Thanks a bunch, part of the goal this time is to keep it simple so I'll definitely do the cathode bias amp. The lightning is a good candidate indeed, thanks for mention that too.

As for trannies wiring diagram I mean the one that tells you all the voltage on every tab, I don't see the voltage in the schematics. They just tell you what to go where. It doesn't help me with what I needed to know or am I missing something here 

[sluckey]

Secondary voltages are on the schematic. But since you have the transformer why not just measure the voltages as suggested earlier?

[Twosteps]

The amp's not with me yet, was just tryin to do homework to save time once I got it. :)

[Backwoods Joe]

Twosteps you might want to give some thought to rebuilding the BJr with the awesome kits that Doug sells here. You've already got the Xformers and it'll be the best sounding BJr you've ever heard.... plus it will be way more versatile than the other amps you mentioned.  $.02

[Twosteps]

Project's down for now.. The pt got 7++ on the filament and the op got 120 ohm on one and 210 on another.. If I'm gonna build some totally new amp into that I might just go back to what I like and do one of the tweed amps.. might be tweed pro I think.

[EL34]

The filament is going to read 7+ without a load on it
That is normal

[Ken Moon]

If you're interested in building a tweed Pro, the 5E5 is a great choice - sort of a "big brother" to the 5E3 Deluxe amp.

I just happen to be selling a very nice Classictone output transformer for this amp, with the proper 6K primary impedance - it can be used with a 2x10, 2x12, 3x10, 1x10, 1x12 or 1x15 speaker setup. The Tweed Pro used a single 15" speaker, but 2x10 is my favorite speaker configuration for this amp.

See the For Sale section for details.

Here is a link to the 5E5 schematic:

http://www.thevintagesound.com/ffg/schem/pro_5e5_schem.gif

Cheers, Ken

[PRR]

The OT reading different _DC_ resistance on each side is normal.

[Twosteps]

@el34 aha, so the pt's probably in the game
@prr right, I'm a little confused now cause sluckey that it should have the same resistance..
How do I make sure that it still good to go? Just don't want to start the project on a shakey ground if you know what I mean..

@ken, yes, the next amp that I would build for myself will be a 5e5. It'll be awhile though.