Considerations when replacing GZ34 with 1N4007's

[trobbins]

360-0-360V secondary is marginal for a single 1N4007 per arm, given mains variation and no load initial start and some margin on a 1kV PIV, and especially if you use a UF4007 (which start to conduct above about 1.2kV under test). I'd recommend 2x 1N4007 in series (or UF4007 in series) to replace each 5AR4 diode side.

The use of back-to-back 1N5408 in the first filter cap neg legs is interesting.

[glebert]

Sounds like someone had a brain fart and wrote DMM but then put in some verbage that would be appropriate with a D'arsonval movement meter, in which you would have to rectify the AC hum to be a DC voltage (which is not an offset, of course).

[Helmholtz]

In the manual, it states: (paraphrased since I can't copy / paste)

"To adjust hum, allow the amplifier to warm up for a few minutes, attach a DMM to the speaker terminals and measure for lowest possible DC offset..."

.

That instruction is plain nonsense, I thought that was obvious.

So if our transofrmer cannot pass DC, how would the hum reach the speaker terminals? Offset is probably the wrong word here, I'm just trying to understand the hum-balance instructions as they seem strange.

Hum reaches the speaker terminals because it's ACV.

[Mr_bibbles]

As an OT can't transfer DCV, there is no DC offset at the output. You might use an AC milliamp-meter though.
But using an ear should do as well.

In the manual, it states: (paraphrased since I can't copy / paste)

"To adjust hum, allow the amplifier to warm up for a few minutes, attach a DMM to the speaker terminals and measure for lowest possible DC offset..."

So if our transofrmer cannot pass DC, how would the hum reach the speaker terminals? Offset is probably the wrong word here, I'm just trying to understand the hum-balance instructions as they seem strange.

[Helmholtz]

Ah, so as you were asking earlier, why would this affect the DC offset at the speaker? It seems like using an ear would be the easier method and effective to boot...

As an OT can't transfer DCV, there is no DC offset at the output. You might use an AC milliamp-meter though.
But using an ear should do as well.

[Mr_bibbles]

Well, it's a simple humdinger (hum balance pot), which uses the cathode voltage for heater elevation.
300B is a direct heated triode, where heater/filament = cathode.

Ah, so as you were asking earlier, why would this affect the DC offset at the speaker? It seems like using an ear would be the easier method and effective to boot...

[Helmholtz]

One curious question though, are any of you experts able to deduce what's going on with that goofy hum adjuster? I have never seen anything like it before.

Well, it's a simple humdinger (hum balance pot), which uses the cathode voltage for heater elevation.
300B is a direct heated triode, where heater/filament = cathode.

[Mr_bibbles]

One curious question though, are any of you experts able to deduce what's going on with that goofy hum adjuster? I have never seen anything like it before.

[Mr_bibbles]

Guys! The issue has been resolved, I detailed in the post above (although I didn't word it as clearly as I should have! I ended up carefully re-flattening the eyelets and then re-tensioning the socket and it worked. I think I was thrown off by the goofy design and the fact that the socket pins wouldn't even reach the tube before the eyelets were straightened out.

These pictures were more for the curious. Thanks again for all the help!

[nevetslab]

OK, so back to interpreting all we have to work with. The ceramic base is 5mm thick. Tube pin length is typically 10mm, so IF the tube is sitting flush on the ceramic base, passing thru the chassis opening, then you have 5mm of pin length left to engage the mechanics of the nice spring-loaded socket terminals. And, you're telling us that isn't long enough to pass thru and fully engage the terminal contacts. While the photos above seem to show there is sufficient space to engage them, there must be some mechanical reason why they're not. The ceramic base is recessed 10mm plus the metal thickness of the chassis (1.5mm). Typical tube base diameter is 33mm, it looks like the 5AR4 I have in my hands would pass thru that opening, and have 5mm to fully engage the contact springs. If they don't, then the mfgr of the tube sockets has no business selling those for that purpose. So, what are we missing here?

[Mick Bailey]

This was covered in post #7

Enzo, the tube base fits flush with the socket base and not the chassis itself; it mates with the socket perfectly. THEN, while it's kissing the plastic, you can see the pins travel underneath and just barely even reach the actual socket pins. It's astounding.

[nevetslab]

Could you please do us all a favor and install the 5AR4 Rectifier tube into this socket, so we can see if it DOES pass thru the chassis opening. If not, and is unable to pass thru to seat onto the tube socket, THEN we will finally understand what it's taken 37 threads to learn.

[Enzo]

And I am guessing the base of the tube doesn't fit through the chassis hole? SO you can never push th tube pins all the way in. The tube in the manual photo looks like it might have been a button base type, which ought to fit.

[Mr_bibbles]

Sorry about the delay here's a bunch of pictures!

[Mr_bibbles]

I'll try and get more pictures soon! The amp is one of two that came in together so should be here for a while. After I re-tensioned the sockets and the eyelets the socket works quite well and even "Snaps" into place with the pins, locking it in without a retainer.