No 5v tap on 100w PT

[studiodunn]

I'm building a 100w Wizard Metal and need to come up with a 5vac source for the relays....5 of them.

This si the PT - http://www.classictone.net/40-18042.pdf


I was wondering if anyone has used this transformer - https://www.alliedelec.com/product/tria ... /70218151/
Id love to avoid spending the $40 for a 5v transformer like amplified parts sells.

[Stevem]

I am afraid you have even bigger issues then that!
If your running 4 EL34 output tubes then they alone need a total of 6 amps of heater current and that PT has only 5.5 amps.

I was going to tell you that if you had some excess heater current to just tap off some filament voltage , rectify that to D.C. And then pump it into a L7806CV positive regulator to end up with 6 volts and 1 amp of current which is still fine for 5 volt relays.

The other thing I noticed unless I am mistaken is that it looks like your PI tube is going to be a mile away from the output tubes, this will produce a uncontrollable osscilating moster of a amp!

[studiodunn]

I am afraid you have even bigger issues then that!
If your running 4 EL34 output tubes then they alone need a total of 6 amps of heater current and that PT has only 5.5 amps.

I was going to tell you that if you had some excess heater current to just tap off some filament voltage , rectify that to D.C. And then pump it into a L7806CV positive regulator to end up with 6 volts and 1 amp of current which is still fine for 5 volt relays.

The other thing I noticed unless I am mistaken is that it looks like your PI tube is going to be a mile away from the output tubes, this will produce a uncontrollable osscilating moster of a amp!

Well that won’t work. Looks like I’m short more than 2 amps for this circuit with that PT. The PI issue isn’t something I had considered since the tie in point to the board was typical in distance from the tube.

I was trying to use some transformers I had on hand but clearly jumped the gun and didn’t do my proper research.

Ok, since I’m obviously the kind of guy that would get this far with the wrong iron, what if I shift the amp to a 5881 build? Or even dumber 6v6s.

Thanks for the help.

[Stevem]

Well if your goal is still to 100 watts under your picking Hand, then 6V6 tubes will not work and on all the Peavey amps that use 5881 tubes and are said to be 120 watt amps ( you know the model I mean) they never under test out even come close to even 95 watts of clean power, no less 120!

Note that if your PI wires to the output tubes from the couplings caps are longer then like 5 inches then your asking for issues from what I have seen.

On some old Gibson amp that had the outout tubes on the right rear of the chassis and the OT on the left side separated by 12 inches they had to resort to sheild wires between the two, so the higher signal level seen on PI wires would have the darn near the same bad effect , or worse!

[pompeiisneaks]

Do you have an OT that matches the impedance needed for the 5881's or the 6V6's?

EL34's like 3.4k for a pair, like 1.7k for a quad, but 6V6 want about 8k a pair or 4k for a quad, and 5881's want about 4k for 2 and 2k for 4...
(those impedances all depend on tube and voltage etc, but that's just to show the general ballpark)

those are highly variable of course, but the point being, often a PT and OT are pretty well matched for a specific tube type, and to change to another, you may need to change one or both at the same time.

The PT you've got per the classictone site, should be for a quad of 6L6/5881, so that should work fine. (It may need power dropping for 6V6?) but if your OT is set for EL34's then you'll get some poor performace with that OT.

What OT are you using?

~Phil

[studiodunn]

Do you have an OT that matches the impedance needed for the 5881's or the 6V6's?

EL34's like 3.4k for a pair, like 1.7k for a quad, but 6V6 want about 8k a pair or 4k for a quad, and 5881's want about 4k for 2 and 2k for 4...
(those impedances all depend on tube and voltage etc, but that's just to show the general ballpark)

those are highly variable of course, but the point being, often a PT and OT are pretty well matched for a specific tube type, and to change to another, you may need to change one or both at the same time.

The PT you've got per the classictone site, should be for a quad of 6L6/5881, so that should work fine. (It may need power dropping for 6V6?) but if your OT is set for EL34's then you'll get some poor performace with that OT.

What OT are you using?

~Phil

My OT is a 1.7K, so I am thinking the 5881s would work.

[pompeiisneaks]

sounds a lot closer, yes. the 6L6GC isn't horribly far off either, it could do about 5.6k for 2 or 2.8k for 4, so it would do okay there, and get closer to the expected 100W as was mentioned by stevem about the others being not close' to the 100W. Not sure if you're caring much about getting 100W exactly.

As far as the 5V then, the 6.3 v could be put into a 5V regulator as you'd only need 900mA per tube for 5881/6L6, and at 4 that's only 3.2A plus the preamp tubes at like 300mA per if I recall? I think you're going to have a few spare mA at that point to run relays... you should do the exact math to be sure though

~Phil

[studiodunn]

sounds a lot closer, yes. the 6L6GC isn't horribly far off either, it could do about 5.6k for 2 or 2.8k for 4, so it would do okay there, and get closer to the expected 100W as was mentioned by stevem about the others being not close' to the 100W. Not sure if you're caring much about getting 100W exactly.

As far as the 5V then, the 6.3 v could be put into a 5V regulator as you'd only need 900mA per tube for 5881/6L6, and at 4 that's only 3.2A plus the preamp tubes at like 300mA per if I recall? I think you're going to have a few spare mA at that point to run relays... you should do the exact math to be sure though

~Phil

Thanks Phil. I can't say that I care about reaching 100w or falling short. I'm only building the 100 watter because I had the iron sitting around and the idea of a 100w Wizard Mtl is insane....

So it's wired up with 5k Screen resistors and 1k grids - Assuming -36v is needed for the EL34s and the -22.5v for the 5881s Do I need to change G2 to 470R?

[pompeiisneaks]

sounds a lot closer, yes. the 6L6GC isn't horribly far off either, it could do about 5.6k for 2 or 2.8k for 4, so it would do okay there, and get closer to the expected 100W as was mentioned by stevem about the others being not close' to the 100W. Not sure if you're caring much about getting 100W exactly.

As far as the 5V then, the 6.3 v could be put into a 5V regulator as you'd only need 900mA per tube for 5881/6L6, and at 4 that's only 3.2A plus the preamp tubes at like 300mA per if I recall? I think you're going to have a few spare mA at that point to run relays... you should do the exact math to be sure though

~Phil

Thanks Phil. I can't say that I care about reaching 100w or falling short. I'm only building the 100 watter because I had the iron sitting around and the idea of a 100w Wizard Mtl is insane....

So it's wired up with 5k Screen resistors and 1k grids - Assuming -36v is needed for the EL34s and the -22.5v for the 5881s Do I need to change G2 to 470R?

To adjust bias voltage you change the bias resistor network, not the grids. The grids at higher values just make your tubes last longer. I don't know the exact schematic you're looking at here, but usually you adjust the dropper resistor in the bias network up or down depending on what you want. You could also potentially add a switch that lets it add in the changed resistance in parallel or series to get you where you cna quickly switch between the two types of tubes. (5881 vs 6L6GC, I think they bias quite differently if I recall). If you're sure you only want one type, then you'd adjust the bias circuit to match. You could probably clone the bias circuit from an AB763 since you're dealing with that type of tube setup and using a PT for that amp, and then figure out how to tweak the resistors in that for the 5881's required negative bias.

Hope that makes sense? You can build it as the above mentioned circuit, and power up w/o the power tubes in and adjust the resistor to see where it gets you. Some here probably know off the top of their heads exactly which resistor to change to a pretty known value for 5881, my brain just never retains that

~Phil

[studiodunn]

Seeing as I got myself into this situation by not paying enough attention I will just stick with 5881's and hope for the best.
The SLO uses a 15k resistor so I may start there. Soldano uses a 2k2 on the screens, but as I have read that there is no audible downside to using a bigger resistor there.
I've also been thinking about setting up the bias voltage on all my new builds with a pot to get the neg-voltage right.

- the schematic I'm using

Wizard MTL50 verified scheme from SLOClone.png

[pompeiisneaks]

Yeah most modern amps have a bias pot to dial in the bias on a per setup basis. It's worth it imo.

~Phil

[studiodunn]

Yeah most modern amps have a bias pot to dial in the bias on a per setup basis. It's worth it imo.

~Phil

Ha! Yes, I put a bias pot on all my builds. I was referring to a pot and leads to establish the proper bias voltage resistor on new builds.

[pompeiisneaks]

Yeah most modern amps have a bias pot to dial in the bias on a per setup basis. It's worth it imo.

~Phil

Ha! Yes, I put a bias pot on all my builds. I was referring to a pot and leads to establish the proper bias voltage resistor on new builds.

oh duh, gotcha, so a tool to figure the right resistor to put in permanently. Gotcha.

~Phil

[studiodunn]

Should I change the grid leak resistors to 100K?

I see the Slo, Tonemaster, ect....5881 amps still use 220, but I've read 100k is the proper value.

[sluckey]

I wonder what Leo would say.