Robinette 6V6 JCM 800 build problem

[bobtait]

So I have put the amp together and all the voltages seem right except the OT is getting full input on the primary but has zero voltage on the secondary to the speaker selector, and there is no sound. Amp is on and heats up I can get voltage on the inputs, all the power supply readings seem right the connections all check out. I can set the bias on the tubes. Just no sound and no voltage from the secondary of a brand new OT. Any ideas anyone?

[JMPGuitars]

Photos, voltages, schematic please.

[bobtait]

Here is the schematic and also photos I sent to mojotone about the output transformer. The amp has 5881 tubes in it at the moment. The voltage going to the OT seemed a bit high.

[bobtait]

The other voltages:
bias circuit hair over 50V
heater to all tubes 6.3v
pin 5 on output tubes to OT 431v
I didn't write down all the preamp voltages but they were all 250v to 300v or so. I was just checking to see if the circuit was active. But I can go and get as specific as needed.

[Bieworm]

Don’t mean to be disrespectful, but this build will likely be very noisy and there are multiple things to address when you get it up and running.
This is what my 800 /plexi build looks like:

[bobtait]

I appreciate your input. If you take away all the wires that look like they are going over the top of everything it looks rather similar to your amp. The only reason they are there for the time being is I wanted to try to connect the bias test points and put bias adjustment pots on the chassis and also try a switchable sag resistor setup and the chassis itself was not laid out to effectively get to those things without going over the top. That said if those things do not prove fruitful and create noise, I will of course remove them. I just wanted to see if I can get away with those added features, and I might not.

[Bieworm]

There’s only one way to find out
The wire from the input hack to the preamp doesn’t look like it's shielded wire though. That will be a problem.
Also, apart from the fact it’s hard to see any similarities between our builds, all of the wires from the board to the sockets should be on the bottom..not flying in the air. Those are actual antennas and will pick up lots of noise

[bobtait]

OK, run the wires under the tagboard and down low. Somehow I was thinking that if they were on the complete opposite side of the amp that the distance of almost touching the other side, close to 3" on this amp would isolate them. But I am trying to learn.

[bobtait]

I need to order some shielded wire for these things. I use Mogami for all the cabling I use and they have a nice smaller wire, I just gotta find the model number for it. Do you have any other recommendations for amp chassis shielded wire?

[bobtait]

So far no one has had an opinion on whether the output transformer might by bad. It is actually kind of funny cause I bought that transformer from mojotone when I thought I had blown the output transformer of british 18 watt bluesbreaker. And I was told, correctly, from this site that it might not be blown. And in the end indeed, that was not the problem. But I still kept it and figured I would use it eventually. And I did on this build. But it has no output voltage when the voltage is everywhere else. Maybe it was bad. Maybe I actually blew the brand new transformer cause I forgot to ground the speaker jack negative before I turned it on. See, I was waiting so long on parts coming in and also the wrong parts coming in that I lost track of where I was in the build. It was on my list to ground the speaker jacks but at the same time. I had fixed another amp that did it different. It grounded the speaker jacks to the ground of filter cap. And that amp is going strong right now. My point is. I am a total newby at this amp building. My experience has always been in performance and production. I actually filled in for a guy at university teaching music majors about live audio production while he was on tour with McCartney. But, that doesn't mean I know anything about the internal workings of these tube amps we love so much. I am depending on you guys to have mercy on me. And show me the best path. I am only on my 3rd and 4th at this point. But I totally appreciate you guys. Sorry for the long winded post.

[Bieworm]

Do you have a scope?

[bobtait]

Yes, I have an O scope and tone generator. I am still learning how to use the scope. I was actually able to find the outer foil on the caps I needed to. Point me in the right direction and I will research how to do it.

[RyanBianchi]

How are you setting the bias on the valves? And what current are you measuring?

What is the switch at the back and those 150Ohm resistors mounted to the chassis for? Your HT voltage is about right if you're using the PT from the schematic with 310-0-310v secondaries.

[bobtait]

Right now it has 5881 valves and I am measuring from pin 5 on the power tubes to ground and setting at just a hair over 50mv at present. The bia and balance adjustment knobs seem to work fine even though they are at the way other end of the chassis and on the other side of the amp. I can get a range of 40mv to 59.5mv. That switch on the back goes from SS rectifier alone to 100 ohm sag resistor to 150 ohm sag resistor. They are the gold metal resistors heat sunk to the chassis if you look at another one of the photos you can see them. I also wanted to use the HT fuse and a Standby switch. All just because they were there on the chassis as well as the switchable impedance. None of these are in the layout I was following. I repurposed that chassis from another amp that didn't work out for me I bought off ebay.

[bobtait]

The PT in the schematic is unavailable. The closest one I could find was a Heybour. If I am spelling that right.

[bobtait]

Here is the other side of the amp if of any use.

[bobtait]

Also another small note. I had my probe, I believe backwards when I measured the bias voltage. I had black on ground and red on pin 5. So I was quoting the opposite reading. If I am correct about that.

[bobtait]

to be honest the Pin 8, Pin 5 thing and bias checking and test probe points got a bit confusing. I couldn't get a good reading on Pin 8, and pin 5 had voltage and the adjustment knobs changed it. So I stuck with that til I could look into it more.

[bobtait]

I went back and rewired my test points from pin 8 which in this design is tied to pin 1. Then through a 1 ohm resistor to the red jacks. And the black jack to ground. I get no voltage reading on that hookup. But, and to correct myself, I am getting -50V on each pin 5 on the power tubes.

[bobtait]

I am realizing that I am learning this stuff the hard way. I get conflicting information searching the net. Sometimes I get stuff that is correct on a cathode bias amp then I find that I was right the first time cause I am working on a fixed bias amp. Sometimes I just have to laugh. But after checking everything again. Proper voltages are present on every tube. The only place it dead, is at the secondary of the output transformer. I am not using the OT spec from the design. I am using a tapped ohm OT from a 18 watt British design. Every one and thing I read said that it would work in this design. And this is where I am at at the moment.