PT for AC30/4 build

[jazrok]

Hi

I'm planning to convert an amp I have into an AC30/4 type amp with just a normal channel (no vibrato). The amp currently has a Hammond 372HX PT in it. It's specs are:

300-0-300 @ 230mA
191VA, 600V @ 230mA, 50V BIAS
Filament 1- 5V C.T.@ 3A, Filament 2 - 6.3V C.T. @ 6A...

Do you think this will work OK for the build?

I realise the B+ voltage is a little higher than ideal. Perhaps I could use a 5u4GB instead of a 5AR4 to lower voltages a bit?

Thanks

James

[Colossal]

Yes, that will run a little hot. Probably about 350VDC under load with a GZ34. The ideal voltage will be around 315-320VDC. A 5U4G may help and you might also consider adding some series resistance to both legs of the secondary, say 50R. Also, some series resistance before B+1, could help pad it down. In a pinch, there is also the use of a zener diode on the center tap to dump voltage.

[sluckey]

I saw your post in the other AC-30.4 thread. Here's a layout I did a couple months ago...

[jazrok]

Thanks guys!

[Roe]

You need to lower the voltage. It's not too difficult since the current draw is relatively stable with an ac30. With a 50R cathode resistor, an ac30 draws 200mA at idle and 250mA at full power.

[Roe]

I saw your post in the other AC-30.4 thread. Here's a layout I did a couple months ago...

looks good but originals used 82R, not 50R - although the latter sounds better to me. Also, higher mains and screens filtering is normally a good idea. Mains should be at least 32uf, screens at least 22uf (max 32 imho)

[jazrok]

looks good but originals used 82R, not 50R - although the latter sounds better to me. Also, higher mains and screens filtering is normally a good idea. Mains should be at least 32uf, screens at least 22uf (max 32 imho)

Ok Thanks. Looks like I should save up for a new PT

[Colossal]

looks good but originals used 82R, not 50R - although the latter sounds better to me. Also, higher mains and screens filtering is normally a good idea. Mains should be at least 32uf, screens at least 22uf (max 32 imho)

Ok Thanks. Looks like I should save up for a new PT

In my opinion, it's best to get the right PT for the task at hand, rather than trying to shoehorn in one that is not going to give you what you need.

[sluckey]

looks good but originals used 82R, not 50R - although the latter sounds better to me. Also, higher mains and screens filtering is normally a good idea. Mains should be at least 32uf, screens at least 22uf (max 32 imho)

This is the AC-30/4 aka AC-34 schematic that I have. Not very clear, but it shows 50Ω, not 82Ω. Can you post an original schematic that shows 82Ω?

[Colossal]

looks good but originals used 82R, not 50R - although the latter sounds better to me. Also, higher mains and screens filtering is normally a good idea. Mains should be at least 32uf, screens at least 22uf (max 32 imho)

This is the AC-30/4 aka AC-34 schematic that I have. Not very clear, but it shows 50Ω, not 82Ω. Can you post an original schematic that shows 82Ω?

Hi Steve,

Refer to this post:
https://ampgarage.com/forum/viewtopic.p ... 99#p417399

There is a modification note on the 1960 AC30/6 schematic which notes the change from the earlier AC30/4 bias resistor of 80R (some had 82R) to 50R. I believe this change was made after the switch from the Haddon PT, which was 285-0-285 to the revised spec of 280-0-280.

[pompeiisneaks]

Yes but he's saying the AC30/4 used the 50, the AC30/6 is what switched, no? Or is that a photo of an AC30/4 with the 82 ohm in it? often those kinds of notes would indicate what was different between the two models right? Or is it indicating that if found the old ones should be modded to the new 'standard'?

~Phil

[Colossal]

Yes but he's saying the AC30/4 used the 50, the AC30/6 is what switched, no? Or is that a photo of an AC30/4 with the 82 ohm in it? often those kinds of notes would indicate what was different between the two models right? Or is it indicating that if found the old ones should be modded to the new 'standard'?

~Phil

No, the AC30/4 used 80R. It was the first model, four inputs. The later AC30/6, six input was 50R. I don't think the 1960 schematic was suggesting that old models be updated to the new spec, only showing a revision history. The early AC30/4s use the Haddon PT which was 285VAC secondary. The later six-input amps changed to 280VAC and the hotter bias.

I have an original AC30/4 schematic and it clearly shows 80R. Also, if you look at photos from early black panel AC30/4s, you can see they have a large, green Welwyn 80R bias resistor.

[sluckey]

No, the AC30/4 used 80R. It was the first model, four inputs. The later AC30/6, six input was 50R. I don't think the 1960 schematic was suggesting that old models be updated to the new spec, only showing a revision history. The early AC30/4s use the Haddon PT which was 285VAC secondary. The later six-input amps changed to 280VAC and the hotter bias.

I have an original AC30/4 schematic and it clearly shows 80R. Also, if you look at photos from early black panel AC30/4s, you can see they have a large, green Welwyn 80R bias resistor.

Thanks for the info. I had read your thread on the AC 30/4 but at the time it didn't register with me about 50Ω v 80Ω. Although I had always wondered why Vox would cook the tubes with a 50Ω.

I'd love to have a copy of your original schematic if you don't mind sharing.

[Roe]

The 50r sounds great, the 80r is disappointing by comparison

[Mark]

It depends on the rail voltages too. I’m using an 82 ohm resistor in an AC-30/6 repair as it has a rail voltage of 350vdc. Using a 50 ohm resistor would simply destroy the output valves.

The output stage is pretty much going flatout at rail voltages of 300vdc, there’s no point adding to its woes.

The 82 ohm resistor sounds fine in grey panel AC-30 amp though the rail is greater than the expected rail voltage.

If I could only get rid of the channel bleed.