Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on June 04, 2020, 06:57:28 pm

Title: JCM 800 Model 2205 voltage chart
Post by: punkykatt on June 04, 2020, 06:57:28 pm

Hello all, Im looking for a voltage chart or a schematic with voltages for the above amp. Can`t find anything on google. This amp has the 5 12ax7`s. Thank you in advance for any help.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: sluckey on June 04, 2020, 07:08:16 pm

Hoffman has the schematic but no voltages.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: punkykatt on June 04, 2020, 07:49:37 pm

Yeah, I know, whats the big secret? Every where I look no voltages on the schematic. My concern is the plate and cathode voltages on V-1 B pin 6 plate and pin 8 cathode. My readings are 60vdc on the plate and .5 vdc on the cathode. Can that be right? The amp seems to play OK.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: SILVERGUN on June 04, 2020, 08:27:59 pm

At 6:56 in this video he gives the poweramp voltages and maybe if you contact him he'll have the preamp voltages. https://www.youtube.com/watch?v=3i8BYwjIxOM (https://www.youtube.com/watch?v=3i8BYwjIxOM)
Never know, most of these YouTubers have to make themselves accessible so you will like and subscribe.
And, I ain't done lookin.

 https://www.linkedin.com/in/craighollabaugh/  (https://www.linkedin.com/in/craighollabaugh/)

Title: Re: JCM 800 Model 2205 voltage chart
Post by: SILVERGUN on June 04, 2020, 08:46:48 pm

Look at reply #11 here:
https://music-electronics-forum.com/showthread.php?t=38597 (https://music-electronics-forum.com/showthread.php?t=38597)
 :thumbsup:

Title: Re: JCM 800 Model 2205 voltage chart
Post by: sluckey on June 04, 2020, 09:13:06 pm

Quote from: punkykatt on June 04, 2020, 07:49:37 pm

Yeah, I know, whats the big secret? Every where I look no voltages on the schematic. My concern is the plate and cathode voltages on V-1 B pin 6 plate and pin 8 cathode. My readings are 60vdc on the plate and .5 vdc on the cathode. Can that be right? The amp seems to play OK.

Plate voltage will be low with a 470K plate resistor. And there's a diode across the cathode resistor that will prevent the cathode from exceeding .6V. I'd say everything is as should be.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: punkykatt on June 05, 2020, 11:45:54 am

Thanks for replies Steve and SG. I found a post from SG`s links where  someone else had voltages similar to what I got. Like Steve says everything is as  should be. Thanks again.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: pdf64 on June 05, 2020, 02:36:46 pm

Quote from: SILVERGUN on June 04, 2020, 08:46:48 pm

Look at reply #11 here:
https://music-electronics-forum.com/showthread.php?t=38597 (https://music-electronics-forum.com/showthread.php?t=38597)
 :thumbsup:

That’s a traumatic read, I remember it from the time  :huh:

Title: Re: JCM 800 Model 2205 voltage chart
Post by: punkykatt on June 05, 2020, 04:26:20 pm

On that thread I scrolled all the way down and found another tread (Appleousa) that is where someone had posted voltages for V1b pin 6 & 3.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: 2deaf on June 05, 2020, 07:29:31 pm

I found some chicken-scratched voltages for a 2210 that I gutted some twenty years ago.  Same preamp as 2205.  I have a stock 2205, but not here at home and I'm not fixing to go down to the studio anytime soon. 

Title: Re: JCM 800 Model 2205 voltage chart
Post by: sluckey on June 05, 2020, 07:36:28 pm

Not quite the same. Your chicken didn't use a 470K plate load and has no diode on the cathode. Makes a big diff in voltages.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: 2deaf on June 05, 2020, 08:56:02 pm

Quote from: sluckey on June 05, 2020, 07:36:28 pm

Not quite the same. Your chicken didn't use a 470K plate load and has no diode on the cathode. Makes a big diff in voltages.

My chicken didn't label the stages, but it did have them point in different directions.  83V on the plate and 0.52V on the cathode. 

Title: Re: JCM 800 Model 2205 voltage chart
Post by: sluckey on June 05, 2020, 09:10:59 pm

Oops! I jumped on the wrong tube.

Title: Re: JCM 800 Model 2205 voltage chart
Post by: punkykatt on June 06, 2020, 09:58:40 am

Thanks 2deaf, much appreciated. I will print that and keep it in my records.