Need help with my first build

[ejendres]

I finished up my first build this morning. Its a 2204 JCM800 clone. I built it using the guts of a Weber Java kit I got really cheap.

Well i fired the amp up to do some testing and while it didn't blow up, something ain't right.

All the preamp and supply cap voltages check out, they're reading a little high but its nothing extreme so I think its just my meter (its a $20 radioshack special )

Where things get weird is the power tubes. The plate is reading around 580v,its suposed to be around 500v. Its not super high, so I'm thinking that reading might be a meter issue.

Where things go crazy is the grid. Its reading a whopping -195v (supposed to be around -35v).

When I measure the cathode voltage it jumps to about 1.5mV but then drops down to 0. Its supposed to be around 35mV.

The heater voltage reads around 4vac, that's low isn't it? The tube definately aren't getting very hot, I was able to touch them after the amp being on for a while.

When I set the bias voltage I would get it to about 45ma, and then it'd just slowly rise higher and higher.

Also there is a faint but distinct background hum coming through the speaker even when the MV is at nothing. And absolutely no sound come through when I plug it in.

So yeah, any ideas? I'm stumped. The good news is i've had the amp on for about 2 hours now with no fires or weird smells, so thats a start.

[toner]

Can you post pictures?

[ejendres]

I can post progress pics to if y'all like.

I actually got it figured out. The bias voltage was obviously way too low at -195. I put a resistor in series with the diode in bias circuit and then I was able to bias the tubes to the proper current. There is pretty bad hum, I think its because I didn't use shielded wires coming off the input and to the gain knob. An easy fix I'll do tomorrow.

Theres a really loud hum when i turn the MV all the way down... any idea what that could be?

The biggest issue is this:


you can hear it on the ring out of the low notes. Sounds like some bad distortion. My amp tech buddy thinks its probably a leaky coupling cap in the preamp. He said to check the grids on the preamp and if the read over a volt or 2 it indicates a leaky cap. I'm gonna try that shortly.

[toner]

You must have the front controls (pots) in a weird order. Are they in this order: presence, master, treble, middle, bass, preamp

That's some nasty distortion. I doubt it's a leaky cap but check V1 pin 2 & 7 and V2 pin 2 for voltage. They should be 0 VDC. If you don't get it figured out, post all your tube pin voltages to help narrow it down.

[ejendres]

I check all those, they were all 0v.

I then check pins 2 and 7 on the PI, they were 5v. They're supposed to be more like 25V. I poked around and found that I had used a 1k resistor instead of a 10k in that part of the circuit.

i swapped it out for a 10k and now there is around 15v at pins 2 and 7. I thought it was fixed but apparently it only made things worse.

http://www.youtube.com/watch?v=FSmBry9ojuo" onclick="window.open(this.href);return false;

[ejendres]

I disconnected the NFB wire and took all the pre amp voltages. They are as follows.

V1: 289, 0, 3.4, 3.9VAC, 3.9VAC, 245, 0, 2.3, 3.9VAC

V2: 190, .008, 1.4, 3.9VAC, 3.9VAC, 365, 190, 199, 3.9VAC

V3: 265, 14.8, 48.4, 3.9VAC, 3.9VAC, 254, 15, 48.4, 3.9VAC

These are check out. So I'm gonna assume the previous distortion issue is solved. But now the amp is motor boating... any ideas on what could be causing this? A video of the amp motorboating is in the previous post.

[toner]

Preamp voltages look fine. V3 grids will read a little low so 15V is fine. What are the power tube voltages and bias?

[ejendres]

For the power tubes the plate for both is around 590, grid about -68, and they're bias to 60 mA.

btw, the tubes are 6550s, hence the high bias.

Now I noticed something weird last night. When I turn the master all the way down the amp starts humming like crazy, but if i touch the chassis the hum mostly goes away and stays away even when I stop touching it.

Also, the treble knob changes the hum. Also, with the treble knob at noon I was able to play fine without any weird noises, but when I turned the treble down it started the low motorboating and when I turned it up it started the nasty distortion.

I checked the voltages at each pot and they all read 0v, so there aren't any leaky caps to the controls.

[51N15T3R]

I don't like the V2 cathode voltage. Maximum heater-cathode voltage for a 12AX7 is 180V, yet you've got 199 at the cathode and 3.9V at the heater (which is a bit higher than it should be as this would put your heater voltage at 7.8VAC), which amounts to a voltage difference of 195V between heater and cathode...well and above the 180V Vkf limit of a 12AX7.

If you're going to run the plate voltage that hot, I would seriously consider elevating the heater supply to a +50VDC reference. Simple to do with a couple of resistors and a 10uF filter cap. Here's a simple schematic to show you how to do it. Should give you a 40-50VDC reference point, which will reduce the Vkf difference on V2 by 40-50V -



What are the AC voltage specs on that transformer?

[ejendres]

This is my tranny's specs

https://taweber.powweb.com/store/022798sch.jpg" onclick="window.open(this.href);return false;

I did not use shielded wires on the input or to the gain knob. I've heard this can cause issues... any chance this is the cause of the blocking distortion? So far there has been any voltages where there shouldn't be as far as the pretubes and the tone controls go so there doesn't seem to be any leaky caps.

[Zipper]

btw, the tubes are 6550s, hence the high bias.

[51N15T3R]

I'll tell you right now that 590V is WAY too hot and that transformer should not be putting that out UNLESS you have the secondary wired incorrectly.

That transformer is set up for a Full Wave Grounded Center Tap rectifier circuit. Red/Yellow is your center tap and should be connected to ground. From red/yellow to each of the reds you should have 360VAC. This voltage, when rectified and filtered unloaded should be around 509V, then should load down roughly about 25-30V down to about 470-480V. 590V is WAY too hot and points to another issue.

Can you post up some more close up pics of the power transformer wiring?

[ejendres]

I can post close ups tomorrow, but the friggin chassis is so small idk if you'll see much.

Here are the progress pics. The issue is that all the wires off the tranny had been cut short when the previous owner of this kit pulled it out of his build. So they're all blue and red wires, I promise you the wires off the tranny are right, I checked those about 5 times. The heater tap is also running really hot, at the source it reads about 3.9vac a side. Maybe its a power transformer issue?

[51N15T3R]

Sounds like you may have a wall voltage issue. Are you running this thing on a Variac? Is there any way you can measure your wall voltage and post up the measurement here?

[ejendres]

Sounds like you may have a wall voltage issue. Are you running this thing on a Variac? Is there any way you can measure your wall voltage and post up the measurement here?

Nah dude, no Variac. I was thinking about this today and I remembered that my meter reads 155VAC off my wall. I think its a meter issue though, because when we lost power with hurricane Irene it also read ~150VAC off the generator. So all the voltages I posted should probably be taken with a grain of salt. My friend's dad is an electrician and I'm gonna have him bring his dad's meter over during the weekend so I can find out whats what.

Nothing smells weird, the tranny doesn't get hot, and the tubes aren't red plating. This makes me think its a meter issue.

Any comment on the blocking distortion? Could it be a biproduct of the unshielded wires?