First time #183 Build

[jam-mill]

SUCCESS!

The 'problem' was following Nik's layout - the color of the OT wires were a swapped vs the TAG schematic.

I built a 50W OTS according to Nik's layout and there was no issue in the power-amp wiring. A comparison of the 183 and OTS 50W layouts indicates that they are the same with respect to the power-amp connections. So, I would contend that there really is no issue with the 183 layout. My 2p.

[MusicMaker]

Don't know why Nik's diagram shows the blue wire going to V4. Could be he uses a different OT??

Here are the voltages.

I measured the bias and the "max" I can get now is 28mV idle current. That's with the pot all the way up.

[martin manning]

SUCCESS!

The 'problem' was following Nik's layout - the color of the OT wires were a swapped vs the TAG schematic.

I built a 50W OTS according to Nik's layout and there was no issue in the power-amp wiring. A comparison of the 183 and OTS 50W layouts indicates that they are the same with respect to the power-amp connections. So, I would contend that there really is no issue with the 183 layout. My 2p.

As Mr. Spock might say, non sequitur. The C-tone 183 layout shows blue and brown OT primaries, which is typical of Fender. This amp uses a Marshall OT, where the leads are usually blue and red, and the phase of the blue lead is the opposite of Fender's blue lead.

[martin manning]

Here are the voltages.

I measured the bias and the "max" I can get now is 28mV idle current. That's with the pot all the way up.

These voltages look good except for the V1 and V2 grids, which should be very nearly zero (like V1 pin 2, e.g.). Maybe your meter is auto-ranging to mV?

To increase the bias voltage you can increase the 1k2 1W on the bias/rectifier board to say 1k8, or reduce the value of the 15k hanging off the bias pot to say 10k. Either one will shift the bias range up a few volts (closer to zero). They're drifting a bit, but the EL34's are still pretty well matched in idle current.

[MusicMaker]

Here are the voltages.

I measured the bias and the "max" I can get now is 28mV idle current. That's with the pot all the way up.

These voltages look good except for the V1 and V2 grids, which should be very nearly zero (like V1 pin 2, e.g.). Maybe your meter is auto-ranging to mV?

To increase the bias voltage you can increase the 1k2 1W on the bias/rectifier board to say 1k8, or reduce the value of the 15k hanging off the bias pot to say 10k. Either one will shift the bias range up a few volts (closer to zero). They're drifting a bit, but the EL34's are still pretty well matched in idle current.

I'll check on getting a couple resistors.

The meter is an old digital ratshack I got many (25?) years ago. I have to put it on mV to get it to read those numbers. It doesn't auto anything. Still works great and the batteries seem to last a long time. So, I suspect that's what I did to 'find' the voltage??

Will adjusting those resistors help with tone and volume controls working?

I still don't have volume, nor do the preamp controls seem to have any effect.
The switches bright/deep/rock/jazz switches also seem to have no effect.

Would the fact that I was in full oscillator mode have caused any component to fail in that area?


I have to say this is a really clean amp. No noise.

[martin manning]

Ok, if those are millivolts then things are good, but you need to write them all in volts or list the units so we know what we're looking at.

Getting the bias dialed in doesn't have anything to do with the preamp controls not working. My guess is there is some wiring error.

At this time you get sound in both OD and Clean modes, correct? Ineffective tone controls makes it sound like you are stuck in PAB mode, and similarly are missing a ground on the volume pot. Maybe start by rechecking the wiring in that section.

I don't think there has been any damage done by the errors found and corrected so far.

[MusicMaker]

I was thinking it wasn't the overdrive as that sounds great.

Probably to the PAB relay? There's no difference hitting the boost and all that preamp stuff is in the area.

I've been a little preoccupied with another project of mine... I'll get to the wiring soon.

[JazzGuitarGimp]

I've been a little preoccupied with another project of mine... I'll get to the wiring soon.

Now we're talkin.... Nice! I'd love to have the skill set to do that.

[norburybrook]

I've been a little preoccupied with another project of mine... I'll get to the wiring soon.

Now we're talkin.... Nice! I'd love to have the skill set to do that.

+1000000000000000000000000


marcus

[MusicMaker]

I've been a little preoccupied with another project of mine... I'll get to the wiring soon.

Now we're talkin.... Nice! I'd love to have the skill set to do that.

I'd love to have the skill set to build amps like Martin does and the rest of you folks!!!


I'm just a hack!

[martin manning]

Lots of diverse talent around here, both on and off the clock.

[MusicMaker]

Update on bias voltages.

Swapped out the 1k2 on the bias board for a 1k7 and my readings for bias are

V4: 35 mV
V5: 36.5 mV
V6: 36.7 mV
V7: 38.6 mV

Will 38.6 be too hot? Should I lower V7 to keep it around 35 mV?
Everything else drops as well if I do. Is there a sweet spot I'm shooting for?

If these are balanced EL34's wouldn't they exhibit the same current? Change the biasing resistors? I wouldn't think wire length would cause the discrepancy. Bad relay switch??

It's still 'breaking in' so I suspect these numbers will change over time.



Still haven't found out why the volume/preamp PAB isn't working. I checked all the wires and it matches Ctones. Continuity tested point to point and down the line, it all checks out. I missed a couple of connections on the switches (little black lines on the sheet) so put those in as well. Are there voltages I should be looking for across the ckt board?


I've also been looking over TAlbany's layout and it's a bit different than Ctones in this area. Is his better than Ctones? On the OD ckt theres the 100k pot and the wiring is 'different'.

Is there any difference between the two in sound?

I'm not opposed to ripping this out and redoing it per theirs.


A little history on the TAlbany layout would be interesting. Why they came up with that layout vs the others?

[Structo]

I would much rather use Tony's layout then C tone....

[MusicMaker]

Good to know. Also, is it possible that when it was an full blown oscillator it could have damaged a component in the ckt? Might explain why no PAB...?

[martin manning]

Update on bias voltages.

Swapped out the 1k2 on the bias board for a 1k7 and my readings for bias are

V4: 35 mV
V5: 36.5 mV
V6: 36.7 mV
V7: 38.6 mV

Will 38.6 be too hot? Should I lower V7 to keep it around 35 mV?
Everything else drops as well if I do. Is there a sweet spot I'm shooting for?

If these are balanced EL34's wouldn't they exhibit the same current? Change the biasing resistors? I wouldn't think wire length would cause the discrepancy. Bad relay switch??

It's still 'breaking in' so I suspect these numbers will change over time.

Matched tubes will have the same static anode current (Ia) given the same applied voltages (Vf, Va, Vg1, Vg2), which is the case in an amp. They should also show the same change in anode current for a given change in g1 voltage (dIa/dVg1), with all other voltages constant. The magnitude of this ratio is the transconductance, or "Gm".

As you probably noticed, you can pay extra for matched tubes, and more extra for matched, burned-in tubes that are less likely to drift. Quality control is not what it was in the era when tubes were dominant, and the matching operations at the various vendors are inconsistent in practices and applied voltages. Some are only matching current and not Gm.

You can only see the static current when setting idle bias, and being within a few mA is close enough. I would swap V4 and V7 to get the total current on each side a little closer to matching, set the average current to ~37 mA and call it good. You have the ability to adjust the dynamic balance with the PI trimmer pot, which you can play with later.

Still haven't found out why the volume/preamp PAB isn't working. I checked all the wires and it matches Ctones. Continuity tested point to point and down the line, it all checks out. I missed a couple of connections on the switches (little black lines on the sheet) so put those in as well. Are there voltages I should be looking for across the ckt board?


I've also been looking over TAlbany's layout and it's a bit different than Ctones in this area. Is his better than Ctones? On the OD ckt theres the 100k pot and the wiring is 'different'.

Is there any difference between the two in sound?

I'm not opposed to ripping this out and redoing it per theirs.


A little history on the TAlbany layout would be interesting. Why they came up with that layout vs the others?

Tony's layout was drawn with knowledge of the real article. C-tone pinched it from TAG and modified it to suit their needs. The OD entrance trimmer has been moved to the rear panel, and the ground scheme has been changed. Otherwise, the circuit looks to be electrically the same.

Did anything change when you added the missing jumpers to the switches (I assume Mid-Boost and Rock/Jazz)? Which controls are working or not working now?

See if the resistance from the volume pot wiper to ground varies as you turn the volume knob.

Confirm that the relays are working by measuring resistance across the common and NO or NC pins with the back panel switches in both positions (foot switch not connected). The pins along each side are coil, common, NC, NO. You only need main power on to do this.