Lil' Bumble-B - 'micro' Bassman with OD channel

[Lynxtrap]

So I decided to jump on the bandwagon and build a "micro". As can be seen it's a Tweed Bassman circuit borrowed fron Rob Robinettes pages.
I also decided to add an OD channel with the hope of getting some Dumble ODS flavour out of it. The kind that is, like "wet" and smooth but can also venture into Marshall territory.

I drew up the circuit basically just estimating component values and such, and added a "tilt" control as a "tonestack" for the OD channel.

Now I got the thing wired up and it makes sound, even great sounds from the Bassman channel.

But the OD channel...actually turned out sounding similar to the dreadful kind that Fender got from their first overdrive attempts in the late 70s and throught the 80s. I think you know what I'm talking about. Elements of a square wave-ish distortion topped with shrill highs, and way too much gain. Popped in a 12AU7, less overdrive but same basic sound.

Now of course I'm gonna experiment with this until it sound like I want i to sound. I have some ideas. But I still wanted to check here before I start throwing resistors and caps at it.

When faced with the problem described here, what would you do or try to achieve a more typical ODS sound out of that channel? The circuit around V2, that is.st
Of course, everything affects everything, but I'm just looking to get that OD channel at lead close to the ballpark without changing the Bass man circuit.

Lil Bumble-B_Rev 5.pdf

[Littlewyan]

Your main issue is the topology is too different. For an ODS style sound you really need that tonestack between the first two gain stages. On top of that the way your gain stages are set up you're not getting anywhere near as much gain as a Dumble.

How much gain do you want? As much as an ODS? If so you'll probably have to change how your gain stages are arranged.

[roberto]

You built a 4 stage lead like Soldano, Bogner, etc..
Well, being the first stage in parallel, it's more like a VHT or Rivera.

If you want to have that amount of bypassed stages, you need to drastically cut lows and highs on every stage.
Coupling caps must be below 1 nF and you need snubbers as well.

If you want a Dumble sound, use one triode from the first stage and then you can have a Vox-like, a clean Dumble-like and a OD Duble-like channels.

[Lynxtrap]

Thanks! I'll experiment a little and see where it ends up.

[roberto]

One more thing I forgot: we are used to the sound of the PI with a 220k load.
If you are looking for some "known" sounds, having a variable load between 600k and 1 MOhm won't sound the same.
You have too much signal going to the ECC99, I'd attenuate it before hitting the PI.

Are you sure you used that PSU scheme? 200+200V fully rectified means 560V, whilst Ua for ECC99 is 400V.
Are then sure of the 2k2 before the anodes of the ECC99?

[jabguit]

200+200V fully rectified means 560V, whilst Ua for ECC99 is 400V.

Huh?

[Lynxtrap]

One more thing I forgot: we are used to the sound of the PI with a 220k load.
If you are looking for some "known" sounds, having a variable load between 600k and 1 MOhm won't sound the same.
You have too much signal going to the ECC99, I'd attenuate it before hitting the PI.

Are you sure you used that PSU scheme? 200+200V fully rectified means 560V, whilst Ua for ECC99 is 400V.
Are then sure of the 2k2 before the anodes of the ECC99?

Well, the whole "clean" channel and the PI is a tweed Bassman circuit (save for the parallell V1) plus a well known PPIMV before the output tube.
The OD channel is meant to be "self-contained" in the sense that it has its own tone control and master volume, and can be set so that it doesn't clip anything downstreams and its volume set to match the clean channel when switching between them.

200V fully rectified is theoretically about 280VDC, but IRL about 300V in this case.

Do you see a problem with the 2.2k resistor?

[Lynxtrap]

Your main issue is the topology is too different. For an ODS style sound you really need that tonestack between the first two gain stages. On top of that the way your gain stages are set up you're not getting anywhere near as much gain as a Dumble.

How much gain do you want? As much as an ODS? If so you'll probably have to change how your gain stages are arranged.

Yes, I realize that the topology is an issue, I was kind of going for what happens in an ODS between the 2nd and 4th gain stage.
As for gain, this thing has a massive amount of gain as far as I can tell, but perhaps the problem is the balance or how and where that gain is utilized?

[FUCHSAUDIO]

"perhaps the problem is the balance or how and where that gain is utilized?"

Bingo.

[roberto]

Well, the whole "clean" channel and the PI is a tweed Bassman circuit (save for the parallell V1) plus a well known PPIMV before the output tube.

The load of the Bassman is 220k, not 1 MOhm. This is what I was pointing at. The response of the PI will be different.
On top of that, are you sure the ECC99 allows for 1 MOhm as grid leak resistor? I remember something around 470k maximum, but I could be wrong.

The OD channel is meant to be "self-contained" in the sense that it has its own tone control and master volume, and can be set so that it doesn't clip anything downstreams and its volume set to match the clean channel when switching between them.

Not really. I mean, yes you an set the volume as you want, but you won't get the "lead" sound if you do it. I'd re-arrange the whole thing TBH.

200V fully rectified is theoretically about 280VDC, but IRL about 300V in this case.

If the trafo is 200Vac you are right, but then you must write it differently on the schematic.
if you have a central tap and you write 200Vac on both ends, it is intended to be 200-0-200V, so 400Vac end to end.

Do you see a problem with the 2.2k resistor?

I expect the power amp to be very soft with that resistor.

[johnnyreece]

200V fully rectified is theoretically about 280VDC, but IRL about 300V in this case.

If the trafo is 200Vac you are right, but then you must write it differently on the schematic.
if you have a central tap and you write 200Vac on both ends, it is intended to be 200-0-200V, so 400Vac end to end.

It appears that the CT is unconnected, and it's wired as a bridge rectifier, which would take .707(ish) times the total voltage (400), which would make it 283-ish volts, unloaded. Feel free to correct me if I'm wrong.

[Ten Over]

From the Tube Town site:

TT-T30-V2
Pri: 0-120V-230V-240V
Sec 1: 200V @ 85mA
Sec 2: 6.3V @ 2A

There is no CT.

[Littlewyan]

What value are you currently using for C3 and C1? .022uF?

Also if you've got a tilt control at the end of your OD channel, do you really want it going back through the last two gain stages of your clean channel and then another tonestack?

You have so many possibilities with this amp that I'm actually jealous I don't have it in my hands! I think we could still get you closer to an ODS sound, but you'll probably have to take one of the gain stages from V1 and move your OD tone control.

[roberto]

It appears that the CT is unconnected, and it's wired as a bridge rectifier, which would take .707(ish) times the total voltage (400), which would make it 283-ish volts, unloaded. Feel free to correct me if I'm wrong.

You are indeed. Try with this if you do not believe me:
https://www.falstad.com/circuit/e-fullrectf.html

Vmax is what we call Vp.

[roberto]

Sec 1: 200V @ 85mA

Ok, so it is wrongly shown on the schematic. It should be 0-200V, not 200-200V.

Sec 2: 6.3V @ 2A

This is another point. 4x 12AX7 plus one ECC99 make 3.2 A.