Matchless/Bad cat build thread

[norburybrook]

First check to see that the path to ground from the grids shows the right resistance, then see if any voltage builds at the grid lug with tubes out and power on to see if you are leaking DC through the coupling caps.

with tubes out and amp off.

Pins 1 0.01R

pins 4 are very variable and don't give a constant reading they're around 10M


Pins 5 both a steady 225k


powered up with no PI or power tubes


I have around 385vDC on pins 3,4 and 6 with 0v on pin 5

I've reinstalled tubes and I've noticed that there no voltage at all on the cathode (or the node with the + and the 360R resistor ) on the schematic it's showing 24v, could this be the cause of the hum and red plating? and what's causing it?

M

[martin manning]

The voltages look good, but pin 1 seems to be grounded, and the 0V o the cathodes with tubes in backs that up. You should see your 360R cathode resistor value there.

[norburybrook]

The voltages look good, but pin 1 seems to be grounded, and the 0V o the cathodes with tubes in backs that up. You should see your 360R cathode resistor value there.

Argh.....I've just realised what I've done.......give me a moment or 2..........

M

[martin manning]

Looks like the node used for the screens differs between the schematic (same as plate at "E") and the C-Tone layout (after the choke, at "S1"), which is more typical.

[norburybrook]

OK, for some dyslexic reason (seeing the ground sign on the layout next to pins 1 and 8 ) and the fact I'm so used to grounding pin 8 on fixed bias amps , I had grounded pins 1 and 8 effectively nullifying the cathode bias!!!

that can be added to my list of stupid mistakes

So, amp sounds pretty good, however the gain channel isn't as gainy as I expected, I thought these amps were high gain from what i'd read, no where near high gain, sort of classic rock gain.

Also it's very noisy , not sure what that's about, the clean channel is fine 'If' I turn down the master volume which is the gain channel only and both channels are running at the same time so the noise from the gain channel is the issue.

I've nothing to compare this amp to but it's definitely too noisy. If I touch the chassis there is a slight drop in ground hum, but the main noise isn't ground issues I don't think. The amp has a single star ground for everything except the speaker jacks/OT.

Also my OT is open on one side as it's a laydown type I've stood up so I was wondering if that's adding to interference as there's no bell covering half of it, however I've seen lots of old tube amps with uncovered Transformers so I don't think that's a problem...other than health and safety in the 21st C.

the other thing I was worried about when I did the installing of hardware was the proximity of the half power and ss/tube recto switches to the output tubes, could this be a source of noise? my thoughts are that if the clean is OK then the whole output section is fine.


So any thoughts on the noisy gain channel and where to start looking?...and if anyone has built one of these amps ;can you elaborate on how 'gainy' the gain should be.


Thanks @Martin for the help again.


M


p.s. my voltages are all down 10-15v would it be worth be using the 230v AC tap on my transformer(I'm using the 240 at the moment) , would that push up the whole rectified voltages of the amp? or would the heaters be an issue?

[martin manning]

Yes, using the 230V primary would raise everything. What is the heater voltage now?

I think you have a bad joint and or bad ground in the gain channel causing the noise. That's based on the fact that you had no gain channel and then it began working.

Clean channel working fine says the power section is ok. What are the voltages on the power tubes now? Looking to check power tube dissipation.

[norburybrook]

Yes, using the 230V primary would raise everything. What is the heater voltage now?

I think you have a bad joint and or bad ground in the gain channel causing the noise. That's based on the fact that you had no gain channel and then it began working.

Clean channel working fine says the power section is ok. What are the voltages on the power tubes now? Looking to check power tube dissipation.

I've chopped sticked everywhere and everything seems fine i.e there's no crackling or noise on any joint /tube pin.

heaters are 3.3v per side.


pins 4 and 5 are 325 and 335 equally per side.


M

[martin manning]

So 6.6 across the heaters. I would leave the primary connections as they are.

Plate voltage (pin 3)? Can you get 4, 6 and 3? Cathode voltage?

[norburybrook]

So 6.6 across the heaters. I would leave the primary connections as they are.

Plate voltage (pin 3)? Can you get 4, 6 and 3? Cathode voltage?

pin 3: 332vDc

pin 4: 327

pin 6: 335

I've reflowed the valve pins and checked the main board again everything seems good but there's definitely something wrong with the gain channel I think, it's just not very gainy. Is this a cascading gain design?

Obviously the clean bypasses the tone stack and the master but with everything on 12 O'clock the gain channel is a lot quieter , I would have thought with the extra gain stages it would be louder , even with the tone stack and master?


also while I have your attention. Is there an easy way to check the OT 4-8-16 taps as I'm not sure I've got them correct. i can't remember how I did them originally when I attached wires and made a label.

M

[martin manning]

What's the power tube cathode voltage?

[norburybrook]

What's the power tube cathode voltage?

23.6 vDC


M

[martin manning]

also while I have your attention. Is there an easy way to check the OT 4-8-16 taps as I'm not sure I've got them correct.

Try measuring resistance from common to each tap as you have assumed them now: Common to 4, Common to 8, and common to 16, and see if 8 is 1.4x the resistance to 4, and 16 is 1.4x the resistance to 8.

For the power stage, 23.6V over 360Ω Rk is 65.5mA, Screen resistor voltage drop is 8V over 1kΩ, which is 8mA. Plate current is then 57.5mA. At 308.4V Va-k that's 17.73W, 17.73/25 = 71%. Pretty comfortable, could be a bit hotter, up to mid-90's.

edit:corrected Va to Va-k

[norburybrook]

also while I have your attention. Is there an easy way to check the OT 4-8-16 taps as I'm not sure I've got them correct.

Try measuring resistance from common to each tap as you have assumed them now: Common to 4, Common to 8, and common to 16, and see if 8 is 1.4x the resistance to 4, and 16 is 1.4x the resistance to 8.

For the power stage, 23.6V over 360Ω Rk is 65.5mA, Screen resistor voltage drop is 8V over 1kΩ, which is 8mA. Plate current is then 57.5mA. At 332V Va that's 19.2W, 19.2/25 = 76%. Pretty comfortable, could be a bit hotter, up to mid-90's.

so with the lower voltage from the transformer I could try the 270R resistors?

I need to get to the bottom of this gain issue though. i remember someone mentioning getting the gain pots backwards. I'm going to investigate that comment.

thanks Martin I appreciate the help as always.

M

[Colossal]

Also, it is hard to tell (and I think I can see the connection...white wire), but is that 220k on the Master connected to the Treble control?

Master.png

[martin manning]

so with the lower voltage from the transformer I could try the 270R resistors?

Yes, but you might have to play a bit. You could tack 1k's across the 360Ω's and get 265Ω, if that's easier to get a data point. A 1W would be big enough for a test.

I need to get to the bottom of this gain issue though. i remember someone mentioning getting the gain pots backwards. I'm going to investigate that comment.

thanks Martin I appreciate the help as always.

Sure, np. You'll get there. Backwards would just be functionally backwards, and touchy. Shouldn't cause loss of signal or noise. I'd try tracing the signal to see where the noise comes in.