6V6 tweed bassman advice

[Bieworm]

Hi guys

recently I built a tweed bassman with a 6V6GT output stage and cathode biased (transformer had no 5V winding and no CT on the HT)
I really like the amp a lot. So for my next build I'm planning to do some tweaks to it.
1st: add reverb
2nd: go fixed bias
3rd: parallel V1 through switching one mixer resistor to ground.

the 3rd thing I'm wondering if the switching won't make a loud pop. If so, do you have any suggestions to counter this? I was thinking about putting a 10M resistor across the switch?

thanks in advance!

Bjorn

[Dave]

I think the switch will probably pop. Not sure if the 10m will fix it, but it might.
I have built one with reverb and it sounds great. You'll love it I'm sure.
Your implementation is fine. I have seem others come up with all sorts of schemes for adding reverb. Seems to me that your implementation makes the most sense, right before the PI just like on a blackface.
Looking forward to seeing how it turns out.
The reverb circuit will have an effect on the overall tone of the circuit. You can play with some values to eliminate some of that. The way I did it was that I "true-bypassed" the entire reverb circuit and then went back and forth with it tweaking values until the tone was very close whether the reverb circuit was engaged or not.


Dave

[tubenit]

I think that is a well thought out idea!   Thanks for sharing the schematic!


With respect, Tubenit

[tdvt]

The reverb circuit will have an effect on the overall tone of the circuit. You can play with some values to eliminate some of that. The way I did it was that I "true-bypassed" the entire reverb circuit and then went back and forth with it tweaking values until the tone was very close whether the reverb circuit was engaged or not.
Dave

Do you remember what values you ended up using?


I have wondered about alternate values to the 3M3/10p standard Fender components (assuming these are the ones you are talking about)

[pdf64]

A couple of negative feedback arrangements look to have been mixed together, resulting in there being far too much. So much that it may oscillate in either polarity, unless presence is turned up.

[Bieworm]

A couple of negative feedback arrangements look to have been mixed together, resulting in there being far too much. So much that it may oscillate in either polarity, unless presence is turned up.

Could you elaborate? Thx

[Dave]

I think it wound up being 1m7/10p for that.


Dave

[shooter]

I'm planning to do some tweaks to it.

do them ONE tweak at a time. test, evaluate, keep or pitch; next tweak, repeat.

[pdf64]

A couple of negative feedback arrangements look to have been mixed together, resulting in there being far too much. So much that it may oscillate in either polarity, unless presence is turned up.

Could you elaborate? Thx

A negative feedback loop takes a sample of the loop output and mixes it back in with the input such that resulting loop gain is reduced.
The sample is a potential divider on the amp’s output, eg 820R and 100R on a regular TR, 27k and 5k on a 5F6A.
So 100/920 x Vo = 0.108Vo is being mixed back in with the power amp loop input.
Or 5/32 x Vo = 0.156Vo


The Brutus has got 820R and 5k, so 5000/5820 x Vo = 0.86Vo.

In the above I’m ignoring the scaling effect if the OT secondary tap used for the feedback sample.

Unfortunately my control theory classes are too long ago for me to remember how too much negative feedback can cause instability.

[Bieworm]

A couple of negative feedback arrangements look to have been mixed together, resulting in there being far too much. So much that it may oscillate in either polarity, unless presence is turned up.

Could you elaborate? Thx

A negative feedback loop takes a sample of the loop output and mixes it back in with the input such that resulting loop gain is reduced.
The sample is a potential divider on the amp’s output, eg 820R and 100R on a regular TR, 27k and 5k on a 5F6A.
So 100/920 x Vo = 0.108Vo is being mixed back in with the power amp loop input.
Or 5/32 x Vo = 0.156Vo


The Brutus has got 820R and 5k, so 5000/5820 x Vo = 0.86Vo.

In the above I’m ignoring the scaling effect if the OT secondary tap used for the feedback sample.

Unfortunately my control theory classes are too long ago for me to remember how too much negative feedback can cause instability.

Great info! Thanks, I’ll take that along for sure!
I think I took the output stage from an AB763 DR, but can’t remember if I altered it. I’m digging this one back up.

[ac427v]

If you use a 300ma power transformer as in the schematic you may find the B+ voltage is too high to use with 6V6GT tubes. And that amp will be HEAVY!

[Bieworm]

If you use a 300ma power transformer as in the schematic you may find the B+ voltage is too high to use with 6V6GT tubes. And that amp will be HEAVY!

That is not my experience with those transformers. Most of my 6V6 amps have a plate voltage of 375DCV (65%bias) with a 5U4 rectifier, a little higher with the 5V4, but not much. The weight is reasonable too.
I have built at least a dozen amps with this transformer. The nice thing is I can run 6L6 or EL34 with the same PTx. The OT that will be used is a 6k 25W type

[Bieworm]

The amp is finished. It sounds pretty good, but the volume is way too low. When I jumper the 3M3/10p the volume goes way up to normal. Is this because of the mosfet somehow? Is there something wrong with the schematic? The volume gets lost in the reverb circuit

[tdvt]

I think it wound up being 1m7/10p for that.

Dave

The 3M3/10p are the components I asked Dave about swapping out, earlier in this thread.

Maybe try his values?

[Bieworm]

I tried something like that. I parallelled a 1M resistor with it, but no big increase of volume.
The strange thing is that the reverb signal is really normal and strong like with a tube, but much lower volume overall.

[tdvt]

Just a thought..


How about lifting one end of the 220K grid leak for V4A to see if that helps?

I have seen that component left out in a few adaptations of the Fender Reverb circuit.

[pdf64]

The amp is finished. It sounds pretty good, but the volume is way too low. When I jumper the 3M3/10p the volume goes way up to normal. Is this because of the mosfet somehow? Is there something wrong with the schematic? The volume gets lost in the reverb circuit

The gain of the added wet - dry mixer stage has been halved, due to its cathode being unbypassed.
Than it’s been halved again by the split anode load resistor.

[Bieworm]

The amp is finished. It sounds pretty good, but the volume is way too low. When I jumper the 3M3/10p the volume goes way up to normal. Is this because of the mosfet somehow? Is there something wrong with the schematic? The volume gets lost in the reverb circuit

The gain of the added wet - dry mixer stage has been halved, due to its cathode being unbypassed.
Than it’s been halved again by the split anode load resistor.

I did try the split load to a single 100k. Didn’t help a lot

[Bieworm]

I have a few thoughts:

1. put a 150uf bypass cap on the 300R resistor on the source leg of the LND150. (Vhttp://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm)

2. go the blues junior route and swap the 3M3/10p for a 470k resistor

3. rewire V3, where V3A will have an IRF820 parralleled and use V3b as a recovery triode

4. move the tremolo to V5a and rewire the PI to cathodyne, rewire the reverb circuit as AB763 type

any other ideas?

[Dave]

Here's an idea... Get rid of the transistor and use the V4a as the reverb recovery and then straight to the PI.


Dave

[pdf64]

The amp is finished. It sounds pretty good, but the volume is way too low. When I jumper the 3M3/10p the volume goes way up to normal… Is there something wrong with the schematic? …

What is the current schematic?
The schematic in the opening post has far too much negative feedback. That will reduce the available system gain significantly.

[pdf64]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

[Bieworm]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

I don’t know if the design is legit to start with. It’s something I assembled from al sources I could find. I don’t know anyone who did the same thing, so I haven’t got a clue if this is a good design.

[pdf64]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

I don’t know if the design is legit to start with. It’s something I assembled from al sources I could find. I don’t know anyone who did the same thing, so I haven’t got a clue if this is a good design.

In reply 5 I highlighted a significant design issue with the negative feedback, did you make any changes in response to that?
If not then the design is not legit, the value of the 820R negative feedback resistor needs increasing significantly, eg try 47k.

[Bieworm]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

I don’t know if the design is legit to start with. It’s something I assembled from al sources I could find. I don’t know anyone who did the same thing, so I haven’t got a clue if this is a good design.

In reply 5 I highlighted a significant design issue with the negative feedback, did you make any changes in response to that?
If not then the design is not legit, the value of the 820R negative feedback resistor needs increasing significantly, eg try 47k.

I’ve put a 33k there, on an on-off switch
But it’s very doubtful it’s related. When I jumper the 3M3/10p and omit the reverb the volume is normal. The loss occurs when the reverb is in the circuit

[pdf64]

…
When I jumper the 3M3/10p and omit the reverb the volume is normal. The loss occurs when the reverb is in the circuit

It doesn’t make sense that you need an additional 30dB of gain to make the amp seem normal. 
So what happens if the reverb mixer stage is taken out of circuit?
ie connect the treble pot wiper to the V5 LTP pin / input cap.
As per a regular 5F6A.

[Bieworm]

I think the switch will probably pop. Not sure if the 10m will fix it, but it might.
I have built one with reverb and it sounds great. You'll love it I'm sure.
Your implementation is fine. I have seem others come up with all sorts of schemes for adding reverb. Seems to me that your implementation makes the most sense, right before the PI just like on a blackface.
Looking forward to seeing how it turns out.
The reverb circuit will have an effect on the overall tone of the circuit. You can play with some values to eliminate some of that. The way I did it was that I "true-bypassed" the entire reverb circuit and then went back and forth with it tweaking values until the tone was very close whether the reverb circuit was engaged or not.


Dave

The switch doesn’t pop, so that’s super!!👍

[RoadShow]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

I don’t know if the design is legit to start with. It’s something I assembled from al sources I could find. I don’t know anyone who did the same thing, so I haven’t got a clue if this is a good design.

In reply 5 I highlighted a significant design issue with the negative feedback, did you make any changes in response to that?
If not then the design is not legit, the value of the 820R negative feedback resistor needs increasing significantly, eg try 47k.

Although I'm still learning, I'd have to lean towards what pdf64 is saying.

Typically, what I've noticed is on the AB763 designs the 820 FB is paired with a 100 ohm at the grounding point.
The 5k pot & cap is what I've noticed on Marshalls with 47k on the 8 ohm xfmr tap or 100k on the 4 ohm or 27k on the 16 ohm tap.
It looks to me like you've mixed the 2 designs of the divider network together.

[Bieworm]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

I don’t know if the design is legit to start with. It’s something I assembled from al sources I could find. I don’t know anyone who did the same thing, so I haven’t got a clue if this is a good design.

In reply 5 I highlighted a significant design issue with the negative feedback, did you make any changes in response to that?
If not then the design is not legit, the value of the 820R negative feedback resistor needs increasing significantly, eg try 47k.

Although I'm still learning, I'd have to lean towards what pdf64 is saying.

Typically, what I've noticed is on the AB763 designs the 820 FB is paired with a 100 ohm at the grounding point.
The 5k pot & cap is what I've noticed on Marshalls with 47k on the 8 ohm xfmr tap or 100k on the 4 ohm or 27k on the 16 ohm tap.
It looks to me like you've mixed the 2 designs of the divider network together.

I don’t agree actually. I have the 33k on the 16 ohm tap and a 5k presence pot. The 1962 bluesbreaker has a 27k on the 16 ohm tap and a 5k presence pot. The PI on both isn’t that different. I can also switch off the NFB, so my volume problem has nothing to do with the NFB. Or am I overlooking something?👍thanks anyhow

[RoadShow]

It’d be good to establish where the gain is being lost, rather than to try lots design changes.

I don’t know if the design is legit to start with. It’s something I assembled from al sources I could find. I don’t know anyone who did the same thing, so I haven’t got a clue if this is a good design.

In reply 5 I highlighted a significant design issue with the negative feedback, did you make any changes in response to that?
If not then the design is not legit, the value of the 820R negative feedback resistor needs increasing significantly, eg try 47k.

Although I'm still learning, I'd have to lean towards what pdf64 is saying.

Typically, what I've noticed is on the AB763 designs the 820 FB is paired with a 100 ohm at the grounding point.
The 5k pot & cap is what I've noticed on Marshalls with 47k on the 8 ohm xfmr tap or 100k on the 4 ohm or 27k on the 16 ohm tap.
It looks to me like you've mixed the 2 designs of the divider network together.

I don’t agree actually. I have the 33k on the 16 ohm tap and a 5k presence pot. The 1962 bluesbreaker has a 27k on the 16 ohm tap and a 5k presence pot. The PI on both isn’t that different. I can also switch off the NFB, so my volume problem has nothing to do with the NFB. Or am I overlooking something?👍thanks anyhow

Missed your 33k comment, was looking at your schematic.

[pdf64]

… I have the 33k on the 16 ohm tap and a 5k presence pot…

As you’ve got problems, it would be a good idea to keep the schematic updated. Otherwise there’s going to be red herrings and confusion.

The 1962 bluesbreaker has a 27k on the 16 ohm tap and a 5k presence pot.

Just to note the JTM45 range has about the highest degree of negative feedback of any valve guitar amp ever.

I can also switch off the NFB

That should make a significant difference, probably over 12dB; does it?
With the negative feedback switched on, the presence control should provide a lot of treble boost as it’s turned up; does it?

The point being ‘is this negative or positive feedback?’

[Bieworm]

Ok.. solved! I’m very very ashamed to say that V2 was pretty dead on 1 triode. I was suspicious about the time it took from switching on until I got sound. Dang!!!

[Bieworm]

Did a quick iPhone recording

eature=shared

[ac427v]

Great tone and nice playing! I will add reverb to my Bassman. Does your schematic show the final version?

[Bieworm]

Great tone and nice playing! I will add reverb to my Bassman. Does your schematic show the final version?

Yes. Except the NFB resistor, which is 33k now.

[ac427v]

Thanks. Do you find the NFB switch to be useful?

[Bieworm]

Thanks. Do you find the NFB switch to be useful?

Yes I do. I Always add it for 2 reasons:
1. For initial startup to switch off the possible OT oscillation
2. I like switching the amp a little raunchier sometimes

[roseblood11]

@Bjorn: Which software do you use to draw schematics? Looks very clean and easy to read.

[Bieworm]

@Bjorn: Which software do you use to draw schematics? Looks very clean and easy to read.

Since I only have a laptop from my employer and I’m prohibited to istall software I draw all my schematics in MS Paint. I invested some time in drawing templates that I can use. Once you’re used to it it’s all right and fast enough