Gregory Mark V tube amp

[M Fowler]

A friend gave me this amp last year and I never did anything with it.

I knew it would need an isolation transformer for saftety reasons so I think I'll order a 120v transformer.

Uses 12AU6, 12AV6, 50C5 and 35W4 tubes.

[M Fowler]

I'm looking at this PT Primary: 115 volt, 60 Hz
Secondary: 115 volt, must be hardwired. VA Rating:35

I wouldn't need a higher VA rating for this small amp I wouldn't think.

[Jack Hester]

Glad you posted this. I'm very familiar with this basic configuration in little Harmony H-303's, Silvertones, and others. But, I'm thinking that this maybe the first time I've seen a Tremolo in one.

I've been playing around with some layouts (drawings, only) of something like this to put in an old tabletop radio that I have. Now, I'm going back and redraw to include a Tremolo. I have lots of these tubes, though I will have to dig around to see if I have a 12AV6. Even if I don't, I can substitute another 12AU6 in the oscillator.

And, this is the first time I've seen a Tone control in this configuration. Please post how it sounds, when you are able to get it up and running. The little speaker won't do it justice. I connected one of my little Harmony's to a 12" Jensen, and it was a really good sounding amp. I know these are all budget amps, but truly underated.

Jack

[Jack Hester]

I CAD'ed your drawing, so that I can now modify as I need for my radio cabinet. I may even leave it as-is (but, with isolation PT).

I've attached the CAD drawing and your original, in one PDF.

Jack

[M Fowler]

Nice work.

I almost ordered the isolation transformer but the shipping was half of the cost of the transformer so in my mind I have to wait until I need more parts to justify the shipping.

[Jack Hester]

On the 12AU6 (V1), I have R3 and R4 labeled with component values of 1Meg, each. The original schematic shows only one value between the two resistors. So, I assumed that they are both the same value. You will have to confirm that for me, when you are able to look at the layout of your amp.

Also, the original shows G3 on the 50C5 as connecting to Chassis Ground. The spec sheet shows it connected internally to K, so that's the way I have it drawn.

Other than these, I believe that I have everything correctly drawn. I tend to draw a bit literally, with actual tube pinouts, rather than that of conventional schematics.

Jack

[Jack Hester]

Here's an updated revision to my drawing, presented as a possible project build.

Found an error on C10. I now show it as an electrolytic, where I did not before.

An isolation PT has been added, and the heater loop has been separated from the now isolated AC feed.

I show a dropping resistor (R16) in the filament loop, but I'm thinking that I may go with a thermistor instead:

http://www.mouser.com/ProductDetail/Ame ... njnnfYk%3d

This thermistor has a total current capacity of 250mA, and the total filament current in this build is 150mA. At ~60% of the total current, I'm thinking maybe a 2 ohm resistance value. I have an AC Ammeter that will read this low current, so if such a build does happen, I can monitor it on the first startup. Along with the total voltage drop across the filaments.

Anyway, take a look and please comment. Have a good one.

Jack

[JazzGuitarGimp]

An isolation PT has been added, and the heater loop has been separated from the now isolated AC feed.Jack

I would offer this caution: if any of the tubes developes a filiment to cathode short, you will lose the isolation provided by the PT. I don't know tubes well enough to say this is a distinct possibility, but thought I'd raise the subject.

[M Fowler]

I don't think I'm going to wire the 12AU6 off primary AC rather then secondary AC (isolation transformer).

[Blindog]

Hey Mark,

I got a bunch of assorted tubes a while back, and your post sparked my memory. I've got two NOS 35W4's and two 50C5's. Let me now if you're interested, maybe we could work out a trade.. I'm in need of some 12AX7's

Mark

[Jack Hester]

Caution well noted.

I completely overlooked what may be the perfect PT for this amp. It has two Secondary 110v outputs rated for 250mA each. That takes care of the filament load and more than enough for the amp. R15 may need a different value, as the original required a rectified 117v to it. My line voltage is 123v to 124v, so I believe that I will be ok with leaving the value alone.

This will eliminate the need for a dropping resistor (R16) or thermistor on the filaments. I use 3A thermistors on a lot of repairs/builds, so I may include one on the front end of this layout. The thermistor that I posted previously may work just fine (1A max rating) for this amp. I like a nice soft start on a cold amp.

Anyway, this configuration allows for total isolation from the incoming line. Please check for errors and comment. Have a good one.

Jack

[Kagliostro]

May be I'm a bit tired

but seems you are feeding AC to the plates

OK. SOLVED, I'm tired

K