JCA22H Board Photographs

[USAPatriot]

No time to really post now, heading off to bed but I wanted to get these up. I took them this afternoon. There's interesting stuff here...the effects loop connections on the left side of V4, an unknown Test Point below V7 and V6...the two violet wires labeled "Bias" and the effects loop wires seen on the jack side of the chassis are just a few neat things to be seen. -Rod-

 

[CTN]

Re: JCA22H Board Photographs

Neat! where do those purple bias wires go?

 

[USAPatriot]

Re: JCA22H Board Photographs

One goes to the main transformer and I think the other one goes to chassis ground. You can see it loop there in front in the 2nd to last photo and one green wire going to ground. Directly across from that green wire is the second wire though you can't see it in the photo. -Rod-

 

[jmh151]

Re: JCA22H Board Photographs

Nice... I'm thinking of getting one of those to replace my Blackstar HT-5

 

[DankStar]

Re: JCA22H Board Photographs

Sorry, can you (or someone) clarify what you discovered and why it's neat? Did you find a new/easier way to bias your amp or something, or does this suggest it's self-biasing?

thanks

 

[Toe-Knee]

Re: JCA22H Board Photographs

the bias pot in the first picture is the best method to bias the amp...

The bias leads are from the Power transformer as has already been mentioned and the other is most likely a center tap.

A method of making it easier would be to install test points on the rear of the chassis along with moving the bias pot to the same location.

 

[USAPatriot]

Re: JCA22H Board Photographs

Sorry, can you (or someone) clarify what you discovered and why it's neat? Did you find a new/easier way to bias your amp or something, or does this suggest it's self-biasing?

thanks

No, actually just filling in what I considered to be a blank when it comes to this amp. After I bought it I went looking for info and really didn't find any. So first I started taking photos and now we're discussing what we see. It's fruitful. -Rod-

 

[Toe-Knee]

Re: JCA22H Board Photographs

No, actually just filling in what I considered to be a blank when it comes to this amp. After I bought it I went looking for info and really didn't find any. So first I started taking photos and now we're discussing what we see. It's fruitful. -Rod-

Photos like these of any amp are a great discussion point.

Especially as the discussion develops it can educate people about their amps and related risks etc.

I learnt a lot of what i know from threads such as this. They're also good as people sometimes spot things that could become an issue and cause damage down the road.

 

[USAPatriot]

Re: JCA22H Board Photographs

the bias pot in the first picture is the best method to bias the amp...

The bias leads are from the Power transformer as has already been mentioned and the other is most likely a center tap.

A method of making it easier would be to install test points on the rear of the chassis along with moving the bias pot to the same location.

Even better would be a small circuitboard with a small LED display that monitors the bias voltage. Just two little numbers that you could see if you looked through the back grill. Actually putting a pot within easy reach might invite too much trouble from people just reaching in a twisting the pot to raise it up. The plus side to having to take the amp apart is that it's a pain, has some hazards so most people just set it and forget it. I wouldn't have a problem moving the stuff myself I guess but I think I'd cut it all out if I sold it. I wouldn't want my mod to allow some kid to crank it up and burn his house down or something, y'know? We like to hammer on B/S and others for making it difficult for us but I think they have sound reasons. lol -Rod-

 

[Toe-Knee]

Re: JCA22H Board Photographs

Even better would be a small circuitboard with a small LED display that monitors the bias voltage. Just two little numbers that you could see if you looked through the back grill. Actually putting a pot within easy reach might invite too much trouble from people just reaching in a twisting the pot to raise it up. The plus side to having to take the amp apart is that it's a pain, has some hazards so most people just set it and forget it. I wouldn't have a problem moving the stuff myself I guess but I think I'd cut it all out if I sold it. I wouldn't want my mod to allow some kid to crank it up and burn his house down or something, y'know? We like to hammer on B/S and others for making it difficult for us but I think they have sound reasons. lol -Rod-

What i do in my amps is do it with a locking pot that can only be adjusted with a screwdriver or with a trimpot like so

I do completely see your concern though. When i make amps for other people i always give them the option and give them a full briefing on how to use it correctly and write up a chart of recommended bias settings for specific valve types.

 

[USAPatriot]

Re: JCA22H Board Photographs

Photos like these of any amp are a great discussion point.

Especially as the discussion develops it can educate people about their amps and related risks etc.

I learnt a lot of what i know from threads such as this. They're also good as people sometimes spot things that could become an issue and cause damage down the road.

Which is really pretty much the point. Those 2 orange relays for instance have near-microscopic printing on them but the part I did work out was the 300DCV part. Pretty much everything in here can bite you and possibly kill you. *I* don't have a problem with using multimeter probes versus a bias probe but I never relax when I'm working and I always make sure my wife is sitting there watching. At least in the same room so she can see me light up and start smoking if it ever happens. All I can say is...don't slip. Put down one probe, make sure it's not going to move then place the other. It's all dangerous stuff. -Rod-

 

[USAPatriot]

Re: JCA22H Board Photographs

What i do in my amps is do it with a locking pot that can only be adjusted with a screwdriver or with a trimpot like so

I do completely see your concern though. When i make amps for other people i always give them the option and give them a full briefing on how to use it correctly and write up a chart of recommended bias settings for specific valve types.

Now that is sweet! Did you have any concerns about the added wire length affecting the readings? Are they measuring voltage or current? The resistors imply voltage. If you measure at the board and repeat at the externalized TP's, they are the same? Thanks. Great amp there! -Rod-

 

[Toe-Knee]

Re: JCA22H Board Photographs

Now that is sweet! Did you have any concerns about the added wire length affecting the readings? Are they measuring voltage or current? The resistors imply voltage. If you measure at the board and repeat at the externalized TP's, they are the same? Thanks. Great amp there! -Rod-

Nah the lengths of the wires are that small that it wont really make an impact whatsoever. They look a lot longer in that picture but in reality they are quite short. The longer wires are just going from the PCB where the trimpot can also be mounted to the alternative placement.

Its measuring the voltage drop from the resistor to ground.

Currently i have it set to either be a reading per valve (50w) or per pair (100w) but in my next build im going to have individual points for each valve and also add seperate controls for each individual valve so you can mix & match valves just so long as the PT can handle the current draw.

And the reading at the board is the actual bias current provided by the PT where as the test points are the actual readings with the load of the valve and the 1R resistor.

Oh also i should add that amp pictured is not actually mine. Its just a good picture example of how i do it. Ive actually moved from using the trimpot now to a locking pot that is fixed directly to the chassis

 

[USAPatriot]

Re: JCA22H Board Photographs

Well, someone did a very neat job, regardless. Thanks for sharing it. I assume that the voltage drop is merely isolating what the tube...and only the tube...is drawing? (versus bias test points that usually have other stuff along the way) The wiring implies that but wanted to be sure my thinking was clear. I'm no tech. -Rod-

 

[Toe-Knee]

Re: JCA22H Board Photographs

Well, someone did a very neat job, regardless. Thanks for sharing it. I assume that the voltage drop is merely isolating what the tube...and only the tube...is drawing? (versus bias test points that usually have other stuff along the way) The wiring implies that but wanted to be sure my thinking was clear. I'm no tech. -Rod-

Yeah its measuring the cathode current for that valve or pair of valves in the case of a 100w etc.

 

[DankStar]

Re: JCA22H Board Photographs

No, actually just filling in what I considered to be a blank when it comes to this amp. After I bought it I went looking for info and really didn't find any. So first I started taking photos and now we're discussing what we see. It's fruitful. -Rod-

Ah, cool