Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on March 03, 2018, 07:27:16 pm

Title: Please Explain
Post by: TIMBO on March 03, 2018, 07:27:16 pm

Hi guys, I have a moody GA40 http://www.ozvalveamps.org/moody.htm that I am restoring..

The usual cap job is done.
Replaced OT that had a blowout on the primary side with a screen resistor fried.

New rewound OT replacement to same specs as old OT with some modern parts.

Fired up well with common 130 ohm cathode resistor on the power valves. Although it had a 120ohm resistor in place.
With the 130ohm resistor the cathode current went to over 100mAs. :huh:
The cathode resistor was adjusted to 366 ohms with stable results.

From the above site the 130 ohm resistor can't be correct????

Also there is a 82k dropping resistor for supply to V1 and V2 this is correct as per my amp.

I'm ASSUMING this was done for a reason?????
(https://i.imgur.com/Bfh0Yr4.jpg)
(https://i.imgur.com/p17tm9P.jpg)
(https://i.imgur.com/txX60mq.jpg)

Amp sounds great with a background fizz so I'll that down to the CC resistors.
The trem is very choppy with a bit added delay.
Reverb, stunning. :icon_biggrin:

 

Title: Re: Please Explain
Post by: PRR on March 04, 2018, 12:27:36 pm

At 400V B+, I would expect 250r common cathode resistor. At this higher B+, 366 seems reasonable.

The 120 may have got copied-over from a EL84 plan by mistake.

Title: Re: Please Explain
Post by: sluckey on March 04, 2018, 12:59:44 pm

Matchless used a separate 270Ω cathode resistor for each EL34/6CA7 in the Chieftain and Clubman. That is equivalent to 135Ω shared cathode resistor. Current and idle dissipation was very hot! Just like what you saw with your Moody. I bet the 130Ω was correct.

http://el34world.com/charts/Schematics/files/matchless/matchless_clubman.pdf

http://el34world.com/charts/Schematics/files/matchless/matchless_chieftan.pdf

Title: Re: Please Explain
Post by: TIMBO on March 04, 2018, 01:01:42 pm

Thanks PRR, I found part of the problem is the amp has a mains voltage adjustment and it was set on 240v.
Our mains these days are around 247v, so the adjustment can go to 250v.I think at some stage our mains were lower.
On the 250v setting the PT output is lower. I'll adjust the cathode resistor for about 90% plate Diss????

I had a mate play the amp and it's got a great sound.
I'm thinking that the low plate voltage on the preamp is correct.
I disconnected the treble cap on the volume pot and it sounded awful, very muddy and uninteresting. Treble cap made it come alive with some drive. 

Title: Re: Please Explain
Post by: TIMBO on March 04, 2018, 01:04:53 pm

Thanks sluckey, Another moody owner has confirmed that the 130 ohm is correct and he has changed the mains voltage adjustment to 250v.

Title: Re: Please Explain
Post by: TIMBO on March 04, 2018, 01:08:23 pm

A bit more to the story here....
http://www.guitargear.net.au/discussion/index.php?topic=49351.0