Pro- amp planning

[Mike-p]

Thanks, that's good to know, I have a second hand weber neomag, paper cap and cloth surround on the way for this which was a bit of luck!

[printer2]

Just doodled some ideas many moons ago, not an exact fit but maybe some ideas would be useful. On the inverters, the 5C5 uses 220k plate resistors in paraphase whereas mine has one pair of 100k's, oops one should be a 82k. I drew it up with a four pole toggle swich changing the inverters and bypassing the gain stage and tone stack of the Bassman. Just utilizing the four pole switch for the inverter the switch can switch between the two as well as having 220k's for the plates (with parallel resistors to drop down to LTP values).
Other than that a cathode bias to grid leak bias on the input.

[Mike-p]

That is really neat thanks, I'd been persuaded against complicated switching arrangements but that didn't look too complicated...argh!

[Mike-p]

Ok, I probably won't implement the PI switch but I'm pursuing it on paper now in order to stretch my brain!

Seems like the variable grid leak /mv would not drop volume to zero as there is always 56k to ground?

The T bolt has the cathodes of the PI tube biased seperately, first 2.2k/33uf, the second 3.9k unbypassed. For the sake of simplicity (ha ha!) should I have them tied together?as in the 5c5 arrangement?

Switching cathode/ fixed bias shouldn't affect things, unless I'm missing something,  your schematic doesn't seem to include power tube bias.

NFB is disconnected when switched to paraphase pi asking with the prescence control. Am I reading this correctly?

Sorry if I'm asking stupid questions or failing to understand simple aspects of the circuits. I really appreciate all the posts on this thread and I feel each time I sit down with my schematic scribblings I have a better understanding of what I'm trying to do.

[tubeswell]

unless I'm missing something,  your schematic doesn't seem to include power tube bias.

output tubes are cathode-bias in that schematic. Bias is bias (and cathode bias is a particular method of biasing). If you want fixed bias on the output tubes, then you have to figure out way of dc-isolating the output tube grids from the preceding driving stage(s) without affecting the way the paraphase inverter attenuates the signal going to the second PI stage's grid. So add more parts

[Mike-p]

Yes sorry you've said this to me previously. The PI switch would work fine in cathode bias? I'm hoping to build the amp simple, add the bells and whistles, play it for a while and work out what I like and then hopefully remove unessassary complications. Accidentally having the paraphase and fixed bias together would not immediately cause bangs and smoke would it?

I can't see any cathode bias resistor and cap on the bassman schematic.

[Mike-p]

What exactly is the resistor from the second half of the PI grid to ground doing. I see it is increased from 12k stock to 56k on printer2 schematic, can I increase it further? I am trying to see if I can provide the attenuated signal to the second half of the paraphase pi whilst keeping my mv/ grid leak arrangement with safety resistors.

[tubeswell]

What exactly is the resistor from the second half of the PI grid to ground doing. I see it is increased from 12k stock to 56k on printer2 schematic, can I increase it further? I am trying to see if I can provide the attenuated signal to the second half of the paraphase pi whilst keeping my mv/ grid leak arrangement with safety resistors.

That 12k resistor is the lower leg of a 270k12k voltage divider that attenuates the signal from the plate of the 1st stage of the PI going into the grid of the 2nd stage of the PI.  Google ‘voltage dividers’ to find out more.


This same voltage divider also doubles as a (270k in series with the 12k =) 282k grid leak resistor for the 6L6 being driven by the 1st triode in the PI.

[Mike-p]

Ok great thanks, that's exactly what I thought so I'm getting somewhere mentally anyway! I'll redraw that section and post the schematic later thankyou.

[tubeswell]

The gain structure of the T-bolt PI is all over the place. 1st triode is a fully bypassed gain stage with a 270k plate resistor and a suboptimal impedance bridge with a 282k grid leak for the following 6L6. 2nd triode also has a 270k plate resistor, but has an input signal that’s been knocked down to 4.2% of the output from the 1st stage, and is also a slightly colder-biased - and unbypassed - gain stage (hence making cathode-current-feedback that further reduces its gain), but it has a better impedance bridge going into its 6L6 than the other side does (because of the 470k grid leak resistor).

To further complicate matters, the shared 200R cathode resistor for the 6L6s is unbypassed, which acts to further reduce gain and ‘even out’ any residual imbalances.



All this is aimed at ‘swings and roundabouts’ gain alterations intended to get a ‘balanced’ push pull output stage. Only it isn’t perfectly balanced, and on top of that, the ‘balance’ becomes more imbalanced as the input signal level to the 1st PI triode increases, which is why it sounds so good.

[Mike-p]

Really appreciate that explanation and I understood most of it I think!

He's my sketch of the PI section with variable grid leak.

[tubeswell]

Looks like you’re going for 2 different PI circuits off the same dual triode. After staring at it for a while until my eyes went all swirly, I’d say you haven’t got enough switches to completely separate the two circuits. Having noted that, the other things I’d be concerned about with this idea are:

1) the switches are going to introduce the possibility of a bit of unwanted noise due to all the different wire runs needed, which would ruin the final experience and satisfaction rating of the build.

2) you’d need one multi-pole switch with a lot of poles to make so everything switched at once. This will be a headache to implement.

3) if you’re intent on this idea it might be a bit simpler (but not much simpler) to build 2 separate PI circuits using separate dual triodes - to cut down on the number of switch poles you need (but you’d still need at least 6 or so switch poles).

4) there’s a certain type of allure in thinking you can have one amp that does lots of tricks by adding switches for Africa but the degree of complexity needed will almost certainly beget lots of troubleshooting grief which the actual experience of can be worse than you might anticipate. So it may be simpler to give up this idea and just build one of each type of amp that you run from an ABY pedal.

Sorry to pour cold water - I don’t want to hurt any feelings. Just trying to help with my honest ‘horses mouth’ suggestions.

YMMV.

[Mike-p]

Thanks, I need cold water in regular doses, I learnt stuff from drawing it out but I think you're right that I'm asking for trouble and misery trying to incorporate it.

[tubeswell]

For your next schematic, may I suggest this straightforward freeware that works on most platforms. Very easy to learn to use and you can build circuits quickly and group components and save these to a folder in the program etc.


http://jschem.bplaced.net/

[8wattjack]

The Pro is one of my all time favorites! In the tweed and black and silver panels. I never had the chance to play a brownie like in Johans video, but imagine it would be similar to my vibroverb or concert. I think you will be really happy with your choice. I just finished a resto on a 65 Pro Reverb and it has the perfect headroom/breakup for moderate volumes on stage. Set it to 10 and use the guitars volume! Love love love these amps!