6V6 ECC83 push-pull amp project

[RogerLLL]

Originally Posted by Silicon

Fuses were sometimes used to connect the centre-tap of the HT secondary winding to chassis.

Are there two diodes in the HT rectifier?

I think I can see four diodes. Are they in series or do the form a bridge?

There are 4 diodes. I've shown these in more detail in an updated schematic which I'll upload shortly.

[RogerLLL]

Here's an updated schematic with all changes incorporated.

[trobbins]

Robert - I missed not seeing a cathode resistor - possibly missed by the OP, but the photo appears to show pin 3 going to the spigot so if that is grounded then something is NQR with that input stage.

[Valvepower]

Hello,

Just had a quick look at the diagram and without ‘mudding’ the water – so to speak – the basic circuit topology and valve count has a similarity to that of the Orange guitar amplifier. I’m wondering if this could offer some sort of a reference. Diagrams attached I located on the Internet.

I hasten to add this offered as a rough guide as the configuration of the 6V6 output sage would still have to be closely based on the one in the original circuit due to transformer ratings etc.

Terry

[Lucien Nunes]

The updated diagram still shows the output grid leaks connected to the cathodes instead of ground, as noted in post #10. But the pic of the underside of the chassis hints at this being the case, which would result in rather a lack of bias. Or is it a trick of the image compression?

[RogerLLL]

That does appear to be how it's wired. Pin 1 in the pic is just used as an additional solder tag for the cathode on pin 8, something I'll have to change given that the 6V6s I was given by Jim are the metal clad versions, as was highlighted by Jim! Would be an easy mod to just connect the grid leaks to ground instead.

[Gabe001]

An 8 ohm speaker on a 4 ohm tap is worse than a 4 ohm speaker on and 8 ohm tap, but very short term unlikely to cause much trouble. the DC resistance of the 8ohm secondary will be marginally higher than that of the 4 ohm secondary.

Pm me if you need the opt.

The 6v6 has a lower screen rating than the 6v6gt (225 vs 285v) so double check the voltages at the pins.

Are you going to respray the chassis or is it just a bit dusty?

[RogerLLL]

It's just a bit dusty, otherwise in really good shape!

[RogerLLL]

Originally Posted by Gabe001

An 8 ohm speaker on a 4 ohm tap is worse than a 4 ohm speaker on and 8 ohm tap, but very short term unlikely to cause much trouble. the DC resistance of the 8ohm secondary will be marginally higher than that of the 4 ohm secondary.

Pm me if you need the opt.

The 6v6 has a lower screen rating than the 6v6gt (225 vs 285v) so double check the voltages at the pins.

Are you going to respray the chassis or is it just a bit dusty?

I notice there are two unused leads from the OT as in the attached photo. Thinking these could potentially be for different output impedances. Is there a way to test this? Thinking resistance would be the obvious one.

[RogerLLL]

Received the ECC83s. I removed the link between pins 1 & 8 on the 6V6s such that pin 8 was just cathode connections and pin 1 to ground.

• I only have 8 ohm speakers available, so I hooked the amp up to the speaker of a 15w Fender Bullet.

• Used a bulb current limiter and fired the amp up.

• No problems with start up, the bulb glowed & then dimmed, so all good there.

• Both inputs work fine, clearly a lot more gain on the first than the second.

• Bass & Treble both sounding great.

• Both volume pots need a clean, and Ch2 pot disconnects at the high volume point.

• Ch1 produces a nice tone with break-up, but thinking the amp as a whole would benefit from a master volume control, as the Ch1 volume is more of a gain control than volume. Is it feasible to place a master volume between V2a & V2b? If not where would be the best location?

• There's some spurious "crackle" in the Ch1 sound. I think this is more than just a dirty pot. I've attached a sound clip which should show this. Could this be the Ch1 coupling cap allowing some DC through? Or is this more likely to be a biasing issue? Or could this be wrong impedance speaker for the output impedance?

• The crackle isn't apparent on Ch2 but this might just be due to it's low volume.

Here's the sound clip:
2023-03-28-11_33_28.zip

[trobbins]

Search the fender and marshal tone stack circuit descriptions and confirm what tone stack you have. Those descriptions will likely also show how a MV is typically added after your tone stack. The 2nd input could be deleted and its vol pot relocated to after the tone stack.

I'd recommend a light spray of contact cleaner, or similar, to the input valve's socket, and then a few repeated insertions to clean surfaces on the valve pins and socket terminals. Also check if the input sockets have shorting switch contacts to ground an input if no plug is inserted - if so then that switch contact also likely needs cleaning. Old pots also typically need a spray of cleaner and some excercise, or worst case a dismantle and clean and then return to normal.

[Gabe001]

not familiar with guitar amps, but I think you've got quite a bit of ringing on sharp transients. Can anyone else confirm in case it's my mobile phone speaker, or maybe that's what the guitar should sound like (breakup?)?

[trobbins]

Did you add the missing cathode bias to the first stage?

[RogerLLL]

I've not changed anything as yet, just wanted to plug it in & see if it worked, how it sounded & any obvious issues. So my next actions will be:

• clean all valve sockets, pots & inputs (these are phono so no switches)

• add cathode bias to the first stage - 2.7k + 100uF electrolytic

• take the 6V6 grid leak resistors to ground rather than to the cathode

• identify the tone stack type

• look at MV options - after the tone stack seems to make sense

• I don't have any use for that Ch2 input as it is, just too low volume, but would like to look at options for switchable clean & overdrive "channels" - wondering if I could move that Ch2 input to the second gain stage 1b, and somehow make it switchable? Something to experiment with.

• work out whether the two unused OT wires are different impedance options (any guidance on how to ascertain this would be most welcome! I've searched online but haven't found anything)

[ms660]

Originally Posted by RogerLLL

• work out whether the two unused OT wires are different impedance options (any guidance on how to ascertain this would be most welcome! I've searched online but haven't found anything)

Do a resistance measurement check between those two wires.

Lawrence.

[Gabe001]

Lots of master volume type mods and useful info here

https://robrobinette.com/5e3_Modific..._Master_Volume

[GrimJosef]

It might also be worth checking that they're connected to the secondary side of the transformer. Depending on where the tranny came from it's not out of the question that it had ultralinear tappings on the primary, which are unused in this circuit.

If they are on the secondary side then the resistances of the windings are likely to be low - of the order of an ohm. If your multimeter struggles to measure resistances in that range accurately then you could try driving a hundred milliamps DC or so through the secondary winding, say from a 1.5V D cell with a 15ohm resistor in series, and then use the 200mV range (say) to measure the voltages between the different tappings. The results should enable you to work out the order of the tappings and the relative resistances between them.

Cheers,

GJ

[RogerLLL]

Originally Posted by ms660

Do a resistance measurement check between those two wires.

Lawrence.

The resistance is 94 ohms - thinking that suggests that they're some sort of additional secondary winding rather than an alternative impedance output?

[Gabe001]

Yes, probably ultralinear taps, on the primary end. Guitar amp builders don't usually like ultralinear amps so you can safely ignore them. You should measure the anode to anode resistance and the lowest unused tap to anode resistance, just to be sure.

[RogerLLL]

Anode to anode is 220 ohms, there's an open loop between both taps & the anode, so presume they're completely out of the circuit.
Is there a way to test the output impedance? Suspect it's not a straightforward process, couldn't fine anything online...