Bassman AB165 unused triod / extra od stage mod?

[Balticnoiseforge]

Hallo, new hobby amateur here!
Currently building a AB165 bassman ( the one with 1 and a 1/2 tube + phase inverter on both normal and bass channel )
Building it pretty much original with some minor modifications in tonestack and so.

But the unused triod, i feel i need something there - but iam not capable of designing a gain stage which would be the last before power amp, and just after the mixer/buffer gain stage.
I will make it bypassable with a dpdt.
Added a image with a gain stage i tried to design, but iam not experienced enough. I basically added a 470pf coupling capacitors before the bypassable dpdt - then tried drawing a cold biased overdrive with a potentiometer to control it when it is switched on. Also having the 470pF and 10meg forming a high pass filter to drive away the edge of the highs. ( Then i realised if gain stage is bypassed the 470pF would connect in series with 0.1uF capacitors at the phase inverter.

Since iam not capable of designing it myself, i ask if someone have done something similiar? Or got some schematic for a gain stage that would work? Iam aiming for something with a little clipping for some extra edge with pot to control it.

If anyone have suggestions Id be happy to look, but will have to study more to design it myself - but some books are on their way!

Fyi: the non working i drew is based of a page in Merlins website with a hint of looking at the dumble preamp, but since it is last in chain it makes it harder. ( If you have a working idea, but do not know if it would sound good in overall circuit that is also fine, Will be bypassable ).

[SILVERGUN]

If it helps, here's a quick snip of howhave I added a stage to a 5E3 with a DPDT switch.

Sorry, I realized this was a pathetic excuse for actual help.

So, I modified your drawing to reflect something I might try:

[Balticnoiseforge]

Yeah the switch is wired correctly for you, where you drew on mine you erase channel mixer if dpdt is grounded.

Here i think i have got all components right, all capacitors are coupled during switching etc - so if i would fill that with correct values it would work i think ( and perhaps sound good )

I need to retain the 220k before the dpdt tho, iam not sure how tube values for a good sound tho.
Changed anode resistor to 120k to bring down voltage abit, 470pf wouldnt change signal alot ( except as it is now with highpass filter ).
If you cant calculate stuff like this - can you add potentiometers and dial it in? Hehe

[Balticnoiseforge]

Or sorry! Just woke!
For some reason i thought the 470k resistors were the channel mixers moved, do not know why haha..
Yeah like you wrote could work except 0.1uf need a DC reference when switch is closed.

[tubenit]

Hot switch (el34world.com)

[pdf64]

I think a 10M or 5M grid leak resistance may be a bit high for cathode bias, may result in a mix of grid leak bias and self bias, making the circuit operating point somewhat dependent on the quirks of the particular valve fitted.
Something in the range of 100k to 1M is more typical.

[acheld]

Is there anything special that you have to do to the input of a paraphrase PI as opposed to a LTP?

[sluckey]

Is there anything special that you have to do to the input of a paraphrase PI as opposed to a LTP?

No. Just treat it the same as any other gain stage.

[Balticnoiseforge]

I think a 10M or 5M grid leak resistance may be a bit high for cathode bias, may result in a mix of grid leak bias and self bias, making the circuit operating point somewhat dependent on the quirks of the particular valve fitted.
Something in the range of 100k to 1M is more typical.

Ab165 got a odd fixed bias, i have changed it to a more normal fixed bias tho.

[Balticnoiseforge]

Hot switch (el34world.com)

I used the dpdt switching like written when i last time hade a bypassable gain stage, it worked just fine and no volume loss at all.

The schematics values are not decided at all tho, Will have to use trimpots and see what works with rnd

[pdf64]

I think a 10M or 5M grid leak resistance may be a bit high for cathode bias, may result in a mix of grid leak bias and self bias, making the circuit operating point somewhat dependent on the quirks of the particular valve fitted.
Something in the range of 100k to 1M is more typical.

Ab165 got a odd fixed bias, i have changed it to a more normal fixed bias tho.

I was referring to the schematic you posted in reply #3, sorry for any confusion.

Also, again with reference to that schematic, with the boost circuit bypassed, the 470pF coupling cap from V3a anode to the power amp’s ‘virtual earth’ input, may have an extreme high pass filter effect. eg the tone will lose a lot of bass, become thin.

[Balticnoiseforge]

I was referring to the schematic you posted in reply #3, sorry for any confusion.

Also, again with reference to that schematic, with the boost circuit bypassed, the 470pF coupling cap from V3a anode to the power amp’s ‘virtual earth’ input, may have an extreme high pass filter effect. eg the tone will lose a lot of bass, become thin.

Yes iam aware of that, having it bypassed would change character of whole amp.
But i do need som coupling capacitor before the switch, obviously i want it to be transparant. But since the circuit topology ( even if values are not right ) i may need some high pass filter, since it is a cold biased / cut off clipper? If you could call it that? Maybe using a 0.01uF capacitator would be better / not Effecting sound while in bypass?

From what i learned i need a resistor to ground at switch tho so it always have a DC reference, else Id rather just have the high pass activated when gain stage is.

If anyone could point me to a good circuit topology Id be grateful, One that dont affect sound in bypass ( but works while engaged ) even if i have to figure out values for resistors myself by trial and error. 120k to anode should bring it down okay tho - Merlins example of the circuit had 100k anode but fed 300v while mine is being fed 330 using that B2+ Wire.

Will attatch Merlins example, its the waveform i fell for, if using a Pot it would almost act as a preamp master volume tho, well not really but abit when engaged ( but feel it might be needed when using such a strong gain stage late in chain, even if it later maybe could be locked.

[Balticnoiseforge]

If i would change the 470pF to 0.1uF while retaining the 10meg DC references / antipop at switches while changing the gridleak resistor to 100k, would that form a highpass filter with 0.1uF before switch giving 15.92Hz? Or should i Keep it 1meg since they do not interact and shape highs after gain stage? ( Would make more Sense since there is why they are formed. )

As people see i usually only mod based on schematics etc, this is not my area

[Balticnoiseforge]

New circuit topology, realised to have a good cold biased half clipper at end of circuit it should biased opposit of all other gain stages, ie 1K5, 25uF.

Also i would need a transparent capacitator before switch + resistor that would not alter sound in bypass.. 0.1uF + 10meg? Will be antipop for switch while not altering sound barely at all right?

[Willabe]

If it helps, here's a quick snip of howhave I added a stage to a 5E3 with a DPDT switch.

You want more gain in a 5E3? 

Volume controls are interactive, use an A/B box, and if that's not enough, use a 12AX7 instead of a 12AY7. Yes it's || not cascaded, but there's plenty of gain.

Neil Young's been doing it for decades and he has a very distorted sound. (And he's got 6L6's in his.)

[SILVERGUN]

If it helps, here's a quick snip of howhave I added a stage to a 5E3 with a DPDT switch.

You want more gain in a 5E3? 

I got lucky. The stage I added was 12AU7 setup a little differently than pictured w/cathode follower driving a tone stack with a separate volume control for the added stage.
The guy I built it for can't decide which way he like it better.