YAOB (yet another one-zero-two build)

[talbany]

Everyone's definition of fizz/fuzz whatever is different..I can tell you the 102 circuit does need to be goosed a bit before the thing starts to breath thicken up and wants to take off..Mine begins around 3 on the master.. Some amps really seem to like the D-lator attached some seem to take off a bit better without it..I have not yet figured out why, could be something to do with the phase angle perhaps..I usually set my master (on the amp) at around 7 seems to help push the loop and give me the best response..If your doing one of these amps this place is an invaluable tool..

Tony

[aflynt]

Hey is it normal for B+ to drop significantly after a few days? It's down to 414VDC right now off 116V AC from the wall socket. I didn't have the foresight to measure wall AC the other day, but the B+ was about 428VDC with the same tubes and everything when I measured it then.

I'm just starting to wonder if this PT is defective. It's been sitting dormant for about 12 years in my closet. The donor amp I used ('73 Quad Reverb) had a different PT ('68 Showman PT) in it for all that time. I'm wondering if I replaced it for a reason back then. I've still got the '68 Showman PT, but the bell is to big to fit in the chassis. How hard is it to remove the bells?

-Aaron

[talbany]

Is it getting hot

T

[aflynt]

I shut the amp off for a bit and then turned it back on and it's back to 424VDC off 116VAC from the wall. The PT gets really hot, but I've found that all these old 85watt Fenders do (at least all the once I've seen). I did notice that the PT was still pretty warm from rehearsal this evening about an hour later.

-Aaron

[aflynt]

The B+ dropped to 418VDC after I left it on for about 10 minutes.

-Aaron

[talbany]

What are the B+ voltage specs on that PT..No load?
You can pull the B+ from the rectifier board measure it with no load cross it with what it should be running
The Math * 1.41 (B+ sec) then allow for 15v or so load drop.. This should be close to what the amp should be running..
If you can't keep your hand on it for longer than 3 sec it's running too hot!!

Tony

[aflynt]

I think it's okay. I left it on for an hour or so came back and it was 428VDC B+ with 118VAC on the wall. I think I'm just seeing the effect of fluctuations in wall voltage.

You were right on about V1 sucking current. I put a different RCA long plate in V1 and the V1 plates went up 10VDC. I wound up switching out the GE I had in V2 with an RCA too since the GE seemed to make things brighter and clean and overdrive were a bit unbalanced tonally.

I still need to get the FET voltages right, though. It's up to 27Volts on the high side and 19 on the low side of the 10k. I had it all biased with two 9V batteries before I wired it in so I'm not quite sure why it's so off. I'm guessing that the ratio is going up with supply voltage. The to 9V batteries were a bit worn so it wasn't quite 18V when I initially biased it.

-Aaron

[ToneMerc]

I still need to get the FET voltages right, though. It's up to 27Volts on the high side and 19 on the low side of the 10k. I had it all biased with two 9V batteries before I wired it in so I'm not quite sure why it's so off. I'm guessing that the ratio is going up with supply voltage. The to 9V batteries were a bit worn so it wasn't quite 18V when I initially biased it.

-Aaron

It's off because you have more than 18V on the "high side", the 18V biasing with the two batteries is not gospel, but serves as a baseline sort of so you can tweak it. It would be spot on if you actually had around 18V at the high side. When using using the 2N4416A my initial drop in build values are 24K and 2K. It sounds like you need to drop that 2.7K slightly.

TM

[talbany]

I think it's okay. I left it on for an hour or so came back and it was 428VDC B+

428 still seems a bit low for a Fender/Showman type.. They generally run 470 no load and are dead on 450 loaded..If it's running hot and 22v under the fuse might be lit on that PT..

Good Luck

Tony

[aflynt]

I think it's okay. I left it on for an hour or so came back and it was 428VDC B+

428 still seems a bit low for a Fender/Showman type.. They generally run 470 no load and are dead on 450 loaded..If it's running hot and 22v under the fuse might be lit on that PT..

Good Luck

Tony

Yeah. That's what I'm worried about. I think I recall the original donor amp ('73 Quad) running super hot and sounding crappy after a set or two. I think that may be why I originally replaced it with a PT from Mojotone (or was it Magic Parts?). Then for some reason I stuck the Mojo (or Magic Parts?) PT in the Showman and put the Showman PT in the Quad. I kind of wish I had a time machine so I could have a brief conversation with myself back in 1998.

I got the bell off the Showman PT, but it's not pretty. Hope I didn't kill it.

-Aaron

[talbany]

Yeah. That's what I'm worried about. I think I recall the original donor amp ('73 Quad)

I played a Silverface quad for years and never really remember the tranny getting to hot on that one..Been a long time though..That was a great amp for me at the time.. Chances are it's the filament winding getting hot as they are usually the first to go..

Tony

[martin manning]

It's off because you have more than 18V on the "high side", the 18V biasing with the two batteries is not gospel, but serves as a baseline sort of so you can tweak it. It would be spot on if you actually had around 18V at the high side. When using using the 2N4416A my initial drop in build values are 24K and 2K. It sounds like you need to drop that 2.7K slightly.

TM

I'd go ahead and put a 2.0k in for the source resistor, that's what I'm using for a 2N4416A too. That will increase the source current and that additional current going through the 150k divider resistor will lower the supply voltage at the top of the 10k. That interdependency is what makes dialing in Vcc at 20V and Vd at 10V difficult. Another way is to clip a pot across your 2k7 and dial it in to get Vd to be half of Vcc. A 2k source will be pretty close, but check again after you get your PT sorted out.

[aflynt]

Okay. I've got a 3k in there right now with a 22k feeding the 10k. I'll try a 2k (hopefully I've got one).

It's kind of funny but despite all this the amp really does sound great. Hopefully it'll just sound even more great once I get it sorted out. About the only thing I've noticed about the amp sonically that is somewhat contrary to my expectations, is that it doesn't really feedback like crazy. I do need to source a PI tube with balanced triodes, though.

-Aaron

[martin manning]

See edit above re using a pot to find a value for Rs.

[aflynt]

If I was to try to find a Varic to use when configuring the amp would a 5 amp rating do? I mean what's the largest current surge you'd expect with one of these?

-Aaron