Restoring a Guild Masteramp

[Gahoo]

Hey all,

The shop I work for brought me an old Guild Masteramp, they brought it up on a variac and said everything works but the tremolo.

I opened it up and am gonna replace the original electros, test the tubes, etc. However, I don't have any experience with amps that run directly off mains voltage. I want to install an isolation transformer and a three-prong to keep it safe and it also has a selenium rectifier which I'd like to replace with silicon.

I'm trying to spec out a good isolation transformer but there's no schematic online or info about what power these amps consume. It's got a 35Z5 rectifier, 2x 12AX7s, a 6Sn7, and 2x 50L6s. What's the best way to calculate power consumption knowing this? My plan was to calculate heater voltage x heater current draw and get a rough idea of the VA requirements for the iso. Just wanted to be sure I was on the right track.

It seemed a little odd to me that the amp would have both a rectifier tube and a selenium rectifier. The selenium is hooked up to the plate of the 35Z5 and runs to the big can filter caps. This is full wave rectification right? I read about how it's necessary to add a resistor after the diodes when replacing a selenium to simulate the voltage drop there which makes sense. Again, I think I need to know the current consumption of the amp to find appropriate diodes and resistor.

Definitely a little out of my depth with this one but I'm looking forward to learning about this style of amp.

Thanks!

[nickb]

Sorry. I know zero about these but I can offer some general advice.

Rather than calculate the power consumption just measure it when driving a dummy load? You can buy kill-a-watt meters that measure watts and more importantly VA for around $15. Also check for a power consumption label on the back.


I would sit down a draw a schematic from what is in front of you and that might address your question about the rectifiers are doing. Even if you can't figure it out there is a good chance that if you post the schematic here then someone will be able to.

[Gahoo]

I was hoping for a way to just calculate a rough figure for VA requirements but if it's something that needs to be measured exactly I can go that route.

Here's a schematic I made for the rectifier section.


Hopefully this is relatively clear, sorry if not.

Leads 5 and 6 of that transformer go off to the pilot light and then to the heaters of he 6SN7 so I'm thinking it's just a filament transformer.

I didn't have a half wave rectifier tube model in Eagle so I drew the plates tied together. The cathode of the 35Z5 goes to the heater of a 50L6.

If it helps someone for clarification on where or how things are connected I can re-draw a section.

[lowell]

Your schematic doesnt show how the filter caps on the right connect to the rectifier. They are grounded, but nothing is feeding them pulsating DC. Clear that up if ya will just to be clear.

You'll want to start with summing the total filament current for all the tubes if you can locate the datasheets. Then find the product of - total filament current * filament voltage. This gives you the filament wattage on the secondary. Be sure to include the rectifier. Then add about 20watts for 2 50L6 push-pull. Then add a few more watts for the preamp tube plates and any power supply bleed resistors...maybe 3-5watts for those.

There's your VA. Hope I didnt miss anything. Probably in the 30VA ballpark.

[nickb]

I'm 100% sure that that sch is incorrect sorry!

Wild guess - if you went for 300VA it would be fine.

Educated guess:-
1) Heaters
35Z5: 35V x 0.15 = 5.25W
50L6: 50V x 0.15 x 2 = 15W
12AX7: 12.6 * 0.15 * 2 =3.75 W
Total: 24W (AC supply)

2) HT for Tubes
12AX7 x 4 @ 0.3W =1.2W
6SN7 X2 @1W = 2W
50L6 x2 @ 10W =20W
Total = 23.2W DC supply
Since this is a DC supply we need to correct by a factor 3.5 for RMS current and for other losses so make it 81VA

Grand total 81 + 24 = 105VA

I guess 300VA might be conservative. 150VA should do it.

Someone had better check my figures!

EDIT: I realised that it a half wave rectifier so had to change the RMS current factor.

[Gahoo]

Here's an updated version of the schematic.



Does this make anything clearer?

Thank you both for the VA calculations but they're not agreeing with each other... @nickb is the plate dissipation wattage really factored in? I was thinking that wasn't cumulative, though I could obviously be wrong.

[nickb]

Here's an updated version of the schematic.

[ATTACH=CONFIG]35702[/ATTACH]

Does this make anything clearer?

Thank you both for the VA calculations but they're not agreeing with each other... @nickb is the plate dissipation wattage really factored in? I was thinking that wasn't cumulative, though I could obviously be wrong.

The plate dissipation IS factored in. Lowell has much the same numbers but didn't allow for the current waveform distortion factor. It's very important.

Those single chip 20W class D amps that use 24W are looking real good right now, huh?

[Gahoo]

No kidding! This amp is about to be about 10 pounds heavier...

[lowell]

Since this is a DC supply we need to correct by a factor 3.5 for RMS current and for other losses so make it 81VA

I wasn't aware of this calculation. Isn't DC already RMS? I can't imagine this PT needs to be more than 30watts for this little amp. Maybe I'm wrong. Aren't 5E3 deluxe PTs 20VA? It's a similar amp albeit 15watts.

[nickb]

I wasn't aware of this calculation. Isn't DC already RMS? I can't imagine this PT needs to be more than 30watts for this little amp. Maybe I'm wrong. Aren't 5E3 deluxe PTs 20VA? It's a similar amp albeit 15watts.

The DC is derived from AC via a half wave rectifier. This produces short current spikes instead of nice sinusoids and as consequence the RMS value of the current is much higher than the DC value.

The Deluxe power transformer is around 100VA, IIRC. Also the Deluxe has a full wave rectifier making the correction much lower ( 2.5* ball park inc losses).

EDIT: *The factor is nearer 1.2, not 2.5 for a FWCT recitfier.

[nickb]

No kidding! This amp is about to be about 10 pounds heavier...

True. And you can't prop open a door with one of those chip-doo-hickies while you're loading all your other gear in the van

[Gahoo]

Is it possible that the 35Z5 rectifies half the wave and the selenium rectifier the other half?

[nickb]

Is it possible that the 35Z5 rectifies half the wave and the selenium rectifier the other half?

I think it's actually a voltage doubler but it's not drawn right. It's dark o'thirty here. Signing off for a bit.

[The Dude]

I believe the selenium rectifier is used for regulation. At any rate, most of what I've read says you should replace it and here's an article on the subject.

w3hwj.com/index_files/RBSelenium2.pdf

[olddawg]

What is it fused with? Take that fuse value and multiply it by 120. That will give you a safe rating for the transformer. Be careful with the selenium rectifier. They are nasty. Usually they are just the equivalent of one diode.