Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MikePR on June 05, 2020, 04:09:55 pm

Title: Single Input JCM 800 2204 clone
Post by: MikePR on June 05, 2020, 04:09:55 pm

Just finished a Mojotone 2204 clone without the Low input jack but it seems I wired something wrong on the cliff jack. Here are some pics. Any idea was wrong? It sounds but with no Gain.

Thx

Title: Re: Single Input JCM 800 2204 clone
Post by: sluckey on June 05, 2020, 04:54:17 pm

Make the corrections shown in red...

Title: Re: Single Input JCM 800 2204 clone
Post by: MikePR on June 05, 2020, 07:08:25 pm

Thx!! That worked. Another question. I want to remove the Presence knob. Could I connect the 4.7k to ground and remove the 22k pot plus the cap? Any recomendation?

Thx

Title: Re: Single Input JCM 800 2204 clone
Post by: sluckey on June 05, 2020, 07:32:34 pm

Quote from: MikePR on June 05, 2020, 07:08:25 pm

Thx!! That worked. Another question. I want to remove the Presence knob. Could I connect the 4.7k to ground and remove the 22k pot plus the cap? Any recomendation?

That'll work. You must keep the 4.7K.

Here's another way to do the same thing. Might be a bit neater...

Title: Re: Single Input JCM 800 2204 clone
Post by: MikePR on June 06, 2020, 04:00:31 pm

Well that one did not worked. There is loud squeal coming from the amp. If i put a 25k pot with the .1 uf cap and 4.7k resistor the squeal only goes away when the pot is full.  :dontknow:

Title: Re: Single Input JCM 800 2204 clone
Post by: sluckey on June 06, 2020, 05:13:47 pm

Quote from: MikePR on June 06, 2020, 04:00:31 pm

Well that one did not worked. There is loud squeal coming from the amp. If i put a 25k pot with the .1 uf cap and 4.7k resistor the squeal only goes away when the pot is full.  :dontknow:

Yes it did, but it reveals that you have the NFB phase wrong. Swap the OT plate leads to correct.

Title: Re: Single Input JCM 800 2204 clone
Post by: MikePR on June 06, 2020, 06:30:05 pm

Thx again! Did worked but had to change the OT pin 3 wires to each other and now is silent.

Title: Re: Single Input JCM 800 2204 clone
Post by: sluckey on June 06, 2020, 07:02:45 pm

Quote

now is silent

Is that good or bad?

Title: Re: Single Input JCM 800 2204 clone
Post by: MikePR on June 07, 2020, 03:03:08 am

Great!!!! 800 Growl. Thx again.

Title: Re: Single Input JCM 800 2204 clone
Post by: MikePR on June 11, 2020, 08:24:06 am

The pilot light flickers every time i turn the amp out of stand by. Meaning on. Is there anything wrong? Do i need to add a cap between pilot light and switch so it does not do it?