100w Bluesmaster- I've made a start

[martin manning]

If a 2A fuse is all you have then you'll have to go with it.

With no tubes in, power up on the limiter. Expect a brief bright glow, followed by a steady dim glow.

Check the plate node, bias supply, relay supply (DC), and filament (AC) voltages to see that they are all present.

Check to see that the bias trimmer works, and set it so the bias voltage is minimum (most negative).

Switch standby to play. Again expect a breif brightening followed by dimming. Check the other power supply node voltages to see that they are all present. They should all be close to the plate voltage, but diminishing slightly as you go down the rail.

If all of the above checks out, switch power off, but leave the standby on for a few minutes to drain the power supply caps through the bleeder on the last preamp node. Connect a load to the speaker jack, and insert the power tubes, leaving the limiter connected.

Switch power and then standby on as above, looking for essentially the same behavior.

If all is still well remove the limiter, power up at full voltage, and check plate and bias voltages.

If these look reasonable, switch standby to play and bring the bias voltage up to get about 65% of max plate dissipation. You'll find out here if you have the OT primaries connected correctly.

Add the preamp tubes and see if you have sound. Recheck voltages, and tweak the bias if needed.

[norburybrook]

thanks Martin,

I'm finished for the day now, it's been a long week.

I'm going to go for it tomorrow with a clear head.


TBC..................



Marcus

[norburybrook]

fired up today, and at first I thought something was broken as there was no light at all from the light bulb and no noise from the transformer. Voltages were there though at the AC heaters and the bias supply also the relays were getting the required voltage which toggled when I threw the switches. 320vDC was there at the standby switch so I switched the standby on, there was a little flash from the bulb then nothing.

Still no hum from the power transformer but plate voltages and B+ were in the over 450v DC range, which I thought was OK. Bias voltage was around -69V .


I've fitted the valves so tomorrow I'll continue.

It's the first amp I've built where there is absolutely NO noise at all from the power transformer, nothing, it's silent, normally there's a hum of varying degrees but these are completely silent!! I'm testing in my studio where the noise floor is very low too. Strange. Maybe it's a good thing a very well built transformer


TBC


Marcus

[martin manning]

What is the wattage of the bulb in your limiter? If it's a100W you won't see much illumination. A 60W will show more. The 450V B+ and -69V bias sound fine; that's with the limiter out, I assume.

A quiet transformer is indeed a well-made transformer.

[norburybrook]

Martin, the bulb is indeed a 100w. The voltages were with the limiter in, no valves, however I'll give accurate readings tomorrow. I quickly just prodded a few choice points to see there were Good DC voltages so didn't take too much detail of the exact readings. I was concerned about the lack of any apparent hum from the PT so was pleased to see healthy DC voltages when I flipped the standby on.


Marcus

[norburybrook]

Ok, well I put the valves in and fired it up...


my 100% success record of OT primaries being the wrong way round is still intact


so no smoke....no sparks....valves not red plating.....


voltages:

Output valves;

430v DC
-37v DC

V1; pins 1 +6 190vDC
3 +8 1.5vDC


V2: pins 1 & 8 212v DC
3 & 8 1.7

V3: pin 1 260 vDC
pin 2 +7 27vDC
pin 3 40 vDC
pin 6 253 vDC.

I've set the bias to around 34-38 mV across the 4 valves


I plugged a guitar in and there's sound but theses something squirrelly going on with the various switching etc. The relays are working but the front controls aren't doing what I would expect. I have a feeling it's the bloody relays again!!

Anyway i'll look into this once the basics are sorted.


cheers

marcus


edit.

Ok, I now realise the clean/ OD relay is working the wrong way round, what do I need to swap to rectify this?

the mid boost seems to working as expected.


clean channel sound great


OD channel is horribly wrong somewhere...there's a massive volume drop and a very thin tone. how do I go about testing the OD circuit?

I will add that I made my own HRM board...a first....

[norburybrook]

I've been over this now for a good few hours and still can't find anything.

here's my OD relay and wiring.

Omron G5V-2-H1 relays, writing on top says 'from bottom'



as I said the switching is the wrong way round for the clean /OD, it defaults to OD.


thanks

marcus

[martin manning]

Looks like you have the NO and NC poles on the OD relay swapped. You need to reverse the yellow and black wires on the bottom right and the OD volume and master volume coax cables. That will fix the reversed OD on/off functionality, but not whatever is wrong with the OD.

I think I would plug a cable into the return jack, and with the OD on and a weak signal on the instrument input jack, use the free end to probe V2a's grid and then V2b's grid to see if the signal gets to and then through V2a. Then probe the HRM treble pot wiper to see if signal gets through V2b. Notice I'm staying clear of the high voltage with the suggested test points. You can touch the V2 terminal strip lugs and the treble wiper eyelet on the HRM board.

A common fault is an errant strand of shield wire grounding out the signal, and there are a number of places where that could occur in the OD stages.

[norburybrook]

thanks martin, I've swapped the relay wires and it's working as expected.

just to clarify your last points.

are you saying; make up a jack cable with just one plug and a wire to the tip and insert it into the return jack, then plug my guitar into the input and on a low level send a signal through while prodding the open end of the insert lead onto the points mentioned?


sorry to be thick, just making sure.


Marcus

p.s. I swapped out the V2 just in case and it's no different so its not the valve.

[martin manning]

Yes correct. Plug something like a signal generator into the input so you have your hands free. A smartphone music player or signal generator would be good. A cable with a plug on both ends will probably work since you should have plenty of room to touch the tip to the test points. Be careful, of course, and keep the signal low 'cause it might get loud.

[norburybrook]

great, I'll get on to it tomorrow. Been recording all day so had no time.

I did manage to chop stick around and couldn't find anything. The OD trim pot is working so the signal is getting there, albeit faintly.


Marcus.

[norburybrook]

OK, did as suggested and I've got good signal to;

V2a's grid and the HRM treble pot wiper but the crappy signal is back again at V2b's grid.


so that tells me something, not sure what at the moment as I need to follow the signal path on a diagram to see where it's getting lost.

I presume that I can check at the end of the HRM board too to see if the signal is getting through that, as that's the thing I made myself and probably has the most potential to be WRONG!!


anyone this side of the pond any ideas before Martin wakes up?



Marcus

[martin manning]

Hmm... What you describe sounds like you have a good signal at the output and a bad signal in the middle. Perhaps the in's and out's are not as they should be? See the pic below so we are sure we are on the same page. The test points are the blue arrows. The treble pot wiper is the output of the HRM stack.

[norburybrook]

Yes that's where I tested. I'm playing a music file through the input as a signal

so V2a from the trim pot has a good full sound.

at the HRM treble wiper the signal sounds eq'd like a high pass filter ( I presume that's the caps and resistor working)it's still strongish though.


at V2b the signal is very thin and weak.


marcus

[martin manning]

The A and B in the maked-up schematic were meant to identify V2A and V2B, but no worries. Seems like you have some problem with V2A or the path from V2A's plate through the OD Drive pot. You could disconnect the coax at R18 and then look for signal on R18's output end to confirm V2A is working. If you have a strong signal there, then its the cabling or the Drive pot.