Blues Junior Rebuild -- PT question

[rdh006]

I want to build something in my unused Blues Junior. Thinking in terms of one of the 18 Watt Lite style layouts with SS rectifier. Looks like this would fit The Blues Jr tube complement.

But the Blues Jr PT has 315 VDC on the secondary, and the 18 Watt info I have seen has shown the PT high voltage at about 290 DC. How do I work out the B+ voltage the B Jr PT will provide, and from there whether I need to drop the B Jr 315V?

If I do need to drop the voltage, how best to do this?

[ric]

Hi rdh,

I see no one's gotten back to you yet. I'm just a hobby builder, not a pro, so I don't want to give out advice on high voltage, but if you search the old threads you should get some answers. I went back and read through all the old design and theory threads and ended up with pages of notes. Good luck with your project.

[rdh006]

Thanks for helping, ric, I appreciate it.

I've done a mass of reading too, but, as a beginner, sometimes it's nice to get some confirmation on the specific thing you're trying to do -- especially with high voltage.

[ric]

Hey rdh,

Yes, I know. I found the Robert Megantz book "Design and Construction of Tube Guitar Amplifiers" to be a good read. It goes over the basics logically, at the same time it takes you step by step through what you need to know to build a Dumble type high gain amp. I havn't built out of his book, but it's there.

I don't know why some people get pages of replies here and others not. I wonder if people tend to follow the crowd. I tend to go AWAY from the crowd! Hang in there, if you want something bad enough and work for it you'll get there. As Neil Young's song says "don't be denied".

Best-ric

[g1]

If the tube complement is the same, the B+ will probably be close, but you won't know exactly til you try it.
If you don't like the trial and error method, but like theory, you'll love this :
http://music-electronics-forum.com/t37277/

As far as lowering B+, the most common method is probably with zeners in series with the CT of the output transformer.
Edit: the zener there will only lower the B+ to the power tube plates. The other voltages can be adjusted through the supply resistor values.

[Justin Thomas]

Hi rdh006,

Getting back to your original question about the B+ voltage, I wouldn't worry too much about 25V too high. Most vintage amps were within spec at +\-20% anyway! If you were starting to push 375, I might worry a little about tubes. I've built 2 2xEL84 amps so far, both run ~350V, nlone cathode biased & one fixed, no failures or tube deaths yet. Just don't bias them so hot that they cook themselves to death. The whole amp works as a system, so if your voltages are a little high, push less current through them... there's always more to it than just one spec (within reason...). Then again, the original tech I took my first build to to troubleshoot, I asked him about my B+ being too high... His reply? "How much were the tubes?" "$20." "Tubes are cheap; just don't risk your Telefunkens in it."

I don't know if it's the case for EL84s, but people have been running 6V6s for decades at nearly 100V over spec. And, maybe he'll jump in, but one guy around here calls EL84s "the bitches of tube design."

I'm assuming you know how to work aorund high voltage?

Justin

[frus]

I want to build something in my unused Blues Junior. Thinking in terms of one of the 18 Watt Lite style layouts with SS and the 18 Watt info I have seen has shown the PT high voltage at about 290 DC

no it doesn't!
the transformer has a 290-0-290 secondary (AC), which, with a tube rectifier and power tubes drawing current, gives the B+ around 320 V

as far as I can see on the schem ( http://support.fender.com/schematics...atic_Rev-D.pdf ), BJ has a B+ of 339V which is in the ballpark and woud make a nice 18W. If yours is 315V, even better

[rdh006]

Thanks for the replies, guys -- very helpful and much appreciated.

Justin, thanks, I'm clear about discharging caps and generally making sure there is nothing left in the chassis. If I have to take measurements, I'm clear about clipping meter leads when possible and using only one hand if I have to probe.

Another question: the PT high voltage has no center tap which I believe means it needs a bridge rectifier. I have previously only worked with tube rectification. I can find plenty of schematics showing a bridge rectifier but I need to translate that to a layout. Does anybody have a relevant picture?

[g1]

Bridges are usually marked at each of the 4 leads. 2 will be marked ~ they go to the HV winding of the PT. One will be marked - which goes to ground, one will be marked + which gives your B+.

[olddawg]

Check out the Ceriatone layouts for an 18 watt light. If you copy it exactly with a turret/tag board, you should have few problems. Just make sure you use the correct pots and jacks. Also new ceramic tube sockets cost about $1 each. A little extra expense and thoughtful planning can save you a lot of headaches later. You could also just buy the pre-made turret board from them I believe.

[rdh006]

Thanks for the replies, once again.

olddawg, I can't find a Ceriatone 18 Watt light layout, specifically, but all the Ceriatone 18 Watt layouts I can find (and in fact all the other 18 watt variants I have seen) use a power transformer with a center tap on the high voltage. The PT I have from the BJr has no center tap. I'm having trouble finding a layout which shows a bridge rectifier, which I believe is what I need to use. The 18 Watt layouts I have seen with SS rectification use a full wave diode rectifier because of the center tap. The BJr layout is for a PCB, and it doesn't show the connections. I'll draw it up myself and maybe I can post it here and get a second opinion or two.

I just realised that the BJr also has an LED pilot light which is run from the filament tap with a dropping resistor and a diode -- I understand the dropping resistor but I don't get the purpose of the diode. The PT is apparently rated for 2.6A for the heater and it already uses 2.4A by my calculations for the 3 x 12AX7 and the 2 x EL84 -- they obviously cut this xformer to the bone for economy -- doesn't seem like there is even room to spare for a different pilot light. So it is this PT which is causing me headaches, really.

[rdh006]

Can anybody confirm whether this looks right?

[rdh006]

Unless I've go it wrong, it looks like it could be done like this also. Can anybody confirm?

[g1]

Sorry, I thought you were using a bridge package. Both your drawings are correct.

[rdh006]

Great. Thanks for your help, g-one.