new build choice, 2nd gen 50w

[norburybrook]

Yep, I got my continuity tester on and worked that out, so when it's in, it's grounding the relay coil.

Just wiring the footswitch now. Had to make a plate for the connector. Didn't want to spend $45 on a switchcraft panel mount connector....

M

[martin manning]

Yep, I got my continuity tester on and worked that out, so when it's in, it's grounding the relay coil

That’s the reverse of what you wanted, isn’t it? That will have FET boost on when it’s pushed in.

[norburybrook]

Martin,

Just fired up for the first time.

I have sound..no smoke/sparks
The FET is working perfectly as expected switch wise except the gain pot is in reverse..need to flip the 1 and 3 pins wiring.

My Overdrive isn't working at all so need to check that over. I had a feeling this would be troublesome due to mixing and matching #40 and #124.

Also the tone stack/switches;

bright switch is working
Jazz/rock working.
Deep switch is doing something..not sure what, which brings me to the tone controls;


treble; working as excepted.... the middle and Bass controls are acting strangely. The Bass seems to have no effect at all unless the mid is turned up quite high..the mid and the bass seem to have a strange interaction, there seems to be too little overall bass compared to say a regular fender/or my Wonderland.

is this normal with this circuit, seems wierd how they interact.

I'll check wiring.

M

edit: bass and mid wires from board wrong way round.

[martin manning]

So the tone controls are sorted? The OD problem must be some wiring error or inadvertent grounding. Pretty good start!

[norburybrook]

Sorted the tone/switches, wiring error from board.

reflowed the OD grids at the valve pins..now working


trying to get the bias values correct.

I've got two 6.8k in parallel on my bias board which should be giving 3.4k I had 27k on the pot but not close enough, changed to a 47k still the highest I can get is -65v.

What do I need to change it to to get me in the 40mv range?

(edit) strange, i've just changed to a 56k pot and the bias voltage is still -65v.

So a 27k,47k and a 56k all give me the same reading. I'm confused.

M

[erwin_ve]

Sorted the tone/switches, wiring error from board.

reflowed the OD grids at the valve pins..now working


trying to get the bias values correct.

I've got two 6.8k in parallel on my bias board which should be giving 3.4k I had 27k on the pot but not close enough, changed to a 47k still the highest I can get is -65v.

What do I need to change it to to get me in the 40mv range?

(edit) strange, i've just changed to a 56k pot and the bias voltage is still -65v.

So a 27k,47k and a 56k all give me the same reading. I'm confused.

M

You're at max bias voltage? The bias voltage is only part of biasing. With the bias voltage you control the amount of current going through the powertubes at idle. Do you use 1 ohm resistors at the cathodes of the powertubes to measure the bias current?

[norburybrook]

sorted it.

1k2 on the rectifier board and 15k on the Pot. Sorted.

all working perfectly now.

Will do a full voltage check tomorrow. I've a feeling the pre amp section is a little on the low side.

Just need to make a footswitch, and then some clothes for it.

And play it in and see if I like it!!


pretty good result really, nothing much wrong.

I'll put up some pics from my phone.








M

[martin manning]

Looks nice! How do you like the FET boost?

[norburybrook]

You know, after years of not having one, It's really nice. It's basically the same as having a nice transparent clean boost available in front of the amp. Something quite a few people do these days with a pedal. Having it on a relay and footswitch is great, really useful.

I was particularly pleased to find I had a matching small knob for the FET gain in my box of tricks from my pedal building days fits perfectly there


I'm going to let it burn in tomorrow and tweak the voltages and perhaps try a 12AX7 in the PI.

what difference does having lower voltages make to the pre amp valves? should I bee looking to raise them up to around 200V with the dropping sting on the power board? I think V1 and 2 are arounf 165 on the plates at the moment.

If I need to up the voltages going to the B+ which way do I need to go with the resistors, I think I'm 2k2-27k-2k2 and 150k on the FET

M

[martin manning]

Yes, I saw the FET knob...perfect! V1 plate voltages around 175-190 are usually best. I don't know what your other voltages are but the easiest thing to do is adjust the resistor just after the choke and bring the whole preamp up or down. Changing to 12AX7 in the PI will raise voltages too, so see where you are when you make that choice. The plate resistors are 12AT7 values... I think I'd change them to 100k/110k if you go 12AX7.

[norburybrook]

Martin,

Yes, I had a feeling that that might change things. I think I'm going to pop a 12AX7 in tomorrow and take it from there.


Thanks for everyone's help again on this one, will be interesting to see if this one stays as a keeper

Fiddly footswitch drilling/soldering tomorrow though. I've bought the wrong size enclosure. it's a little on the small size for two switches. I'm going to build it anyway, I don't have big feet.


M

[norburybrook]

I just swapped the EHX 12 AT7 out for an unbranded Chinese 12AX7 I had in a box.....WOW ..what a difference. Amp has sprung into life all round, it's fuller, the tone controls and switches all seem to work much better now.

I've got a 413v DC coming off the rectifier board. and with my dropping string I'm seeing

v1; 174
v2; 169
V3; 314

The JJ 6L6's are the best match I've ever had from a pair of valves...they're within a few tens mV of each other. I have it biased at 44mv.

I'm not touching anything at the moment as it sounds good. Unfortunately I'm using my EVM 12 cab...i don't want to put an EVM12 in a combo....argh!! let's see when I build the cab what works best. it's a shame there isn't a 'neo' EVM12 L

Time to build a footswitch.

M

[erwin_ve]

Hi Marcus, Glad you got it working! Looks are great.
As for the "sprung into life" you might consider: did it because the voltage at the pre amp did become higher overall or is it in the PI tube alone?
I'm curious because voltages generally means, higher voltage ; more headroom, more highs, stiffer feel.
Lower voltages: darker tone, less headroom, looser feel.

[norburybrook]

Hi Marcus, Glad you got it working! Looks are great.
As for the "sprung into life" you might consider: did it because the voltage at the pre amp did become higher overall or is it in the PI tube alone?
I'm curious because voltages generally means, higher voltage ; more headroom, more highs, stiffer feel.
Lower voltages: darker tone, less headroom, looser feel.

Erwin,

I had similar thoughts. I think it's not just a voltage thing, as I've had amps before where changing the pre voltages didn't make as much difference as I'd thought.maybe with a lot of these things if you're running the amp flat out it might do but who in reality ever does that?

I could swap the first dropping string resistor and try and get another 20v overall and report back.

Anyone care to tell me what I'd need to change the first resistor to from 2k2 to get a higher overall voltage down the line?

M

[erwin_ve]

I had similar thoughts. I think it's not just a voltage thing, as I've had amps before where changing the pre voltages didn't make as much difference as I'd thought.maybe with a lot of these things if you're running the amp flat out it might do but who in reality ever does that?

I could swap the first dropping string resistor and try and get another 20v overall and report back.

Anyone care to tell me what I'd need to change the first resistor to from 2k2 to get a higher overall voltage down the line?

M

Running flat out has IMO little to do with it. If your v1 on the anode is 160V with the 12AT7 installed on v3, and it's 175v on the anode on v1 with the 12AX7 on v3, you're gonna hear that. I'm not saying you should alter anything it's more a curiosity thing.

Is it possible for you to do a measurement with the 12at7 on v3?

For the dropping string you could try a 1k8 resistor.
Edit: when doing this you probably have to bias again.