What mods have been done to my 2204?

[BrianPhase90]

Hi guys, long time listener first time caller

It's been modded... It runs EL34's now... But I think it wasn't always that way.. There are big 10W resistors installed between one of the power tube socket pins and ground..

There are also resistors/caps added to existing resistors on the PCB (in parallel)

Does anyone know what they did here? Should I reverse it? I originally opened up the chassis to tighten the input jack and found this... Help? :sadwave:


ALSO one of the resistors has broken loose from one of the power tube socket pins... Whats going on here?

http://i281.photobucket.com/albums/kk22 ... 0_2080.jpg
http://i281.photobucket.com/albums/kk22 ... 0_2083.jpg

[elda]

[quote]There are big 10W resistors installed between one of the power tube socket pins and ground../quote]

AFAIK, 1 watt/ 1 ohm resistors are added (connection to pins 1/8 on the power tube sockets to ground) to help attain accurate bais readings. In the picture I can't tell what pins the 10 watt resistors are connected to. Do you know what the resistance values are on these 10 watters

AFA the board, I cant tell what is going on there.

How does the amp sound? Are their any issues while playing the amp?

[BrianPhase90]

I don't believe that there are any issues. The resistors are indeed 1Ohm and from pin 8 to ground... So this is just a normal deal here?

[elda]

From what I can tell the resistors on the power tube sockets that you mention were added to aid in attaining accurate bias readings. I just opened my 4010 (112 2204) to see the power tube sockets. There are no resistors, just a straight connection to ground.

IMO, if the amp is playing fine and you are OK with the tone I would leave it as is. However I know from personal experience things won't sound and feel right because you know that "x" and "Y" are not what they should be and the urge to change them back to the correct spec can be overwhelming.
Here is a link to some info on JCM800's Maybe this can help regarding the mods on your board. Hope this helps.
http://www.tone-lizard.com/Ultimate_JCM800.htm

[elda]

Took a closer look at the pic you posted of your 800's board.
Did some research and I ascertain the following:

1. C1 on the board, which is a cathode cap has been removed and replaced with an electrolytic axial cap. The stock value for this cap is a non polarized .68 uf. From the pic I can't tell what the value is of the new cap.

2. C4 on the board which is a 470 pf cap that works with R5 (a 470K resistor)as a treble bleed has been removed. Many 800 owners typically remove this "bright" cap in an effort to tame the ice pick characteristics of the amp. IMO a good upgrade to the tone of the amp

3. It appears that diodes were added at R5 (470K resistor) I have never heard or seen this mod. Not sure what it does. If this were my amp, I would probably remove them, simply because I don' know what the diodes are doing.

4. C5 on the board, a .001 uf cap, has been removed. This is a "bright" cap that is connected to the preamp volume pot. This is also a cap that many 800 owner remove, in order to tame the ice pick characteristics of the amp. IMO, this is a great upgrade to the tone of the amp.

5. C8 on the board, which is a 470pf cap has been removed. This cap works in conjunction with the mixer resistor R10 (470K). The removal of this cap "opens up" the overall ton of the amp. IMO another upgrade to the amps tone.

6. A cap has been added (parallel) to the R9 an 820 cathode resistor. Can't tell what the value is of the cap, but typically this is a mod that utilizes a .68uf cap that is will increase gain and will give a "plexi" mid range boost. This is a mod that I will be doing to my amp.

With the exception of the addition of diodes to the resistor at R5 and the removal and replacement of the cap at C1, I have done most of these upgrades to my 4010 (112 2204) and have had great results.

Hope this helps

[demonufo]

Personally I disagree with that a little, regarding C4 and C8. I feel that these amps (2203/2204 et al.) work much better with both these intact. However, C4 as a 250pF cap is quite nice too. I found I lost a little too much definition without either of these caps.

C1 really ought to be replaced with a decent quality polyester film type. I'm fairly certain these are the same type and series as the original caps, albeit made in a different colour;
http://uk.rs-online.com/web/search/sear ... &R=0117785

Not sure what those diodes are acheiving.

[Dai]

*IMO* the person who carried out the mods probably didn't particularly know what they were doing. For one a 10watt(or whatever that is) 1 ohm resistor to help read the current draw is utter overkill. Sort of akin to bringing a keg size container to a picnic when all you need is a thermos bottle.

[Steve-Grindrod]

The capacitor accross R1 originally was a 0.68uF and been replaced with a larger value electrolytic, value not determinable from the photo.
This will increase the low end by whatever value it is. Depending on what you like/don't like in your sound, and what type of guitar you use, it can be a good or a bad thing. It certainly would help to fatten up single coils.
BUT, it should be pointed out that it has been fitted the wrong way round! If you decide to keep it, unsolder it and turn it through 180 degrees! At the moment the cap +ve is going to ground and needs to be going to the V1a cathode (the yellow wire).

The 2 back to back diodes added across R5 470K are acting like a very primitive noise gate. Although I say primitive it is actually quite a useful little device. Basically the diodes won't let through any signal below the threshold voltage of the diodes (approx 0.6V pk-pk) which compared to the signal levels in a valve amp is relatively insignificant when the amp is at full bore. The downside is when you are trying to play cleanly and relatively quietly - the diodes will introduce some distortion as they try to conduct, not disimilar to cross-over distortion. It gets lost when the amp is cranked.

The other capacitors that have been taken out, as pointed out, can be left out or put back in, it really is a matter of personal taste. when I designed it way back when I liked it and it certainly suited the "tone of the times". probably these days I would do something different - but that's the benefit of 30 years time difference.

What does concern me is the 10watt resistors on the output valve bases. Absolutely nothing wrong in having the resistors there, it helps setting up the bias - BUT the size of those 10watt devices puts them dangerously near the high voltage supplies! fortunately if they touch the HT fuse will instantly short - but do you want that happening at a gig?

Steve

[BrianPhase90]

Thanks for all the replies!

[Miracle Man]

Brian, can you check what the value of that blue capacitor on the far left is?
Thanks