Tube Town B-Flex

Looking closer at this I see two (of possibly several) options:

Option #1:
GAStan said:
I like your original suggestion of replacing R14 and R15 with a 1m pot. It changes the circuit very little and gives the volume control you desire. I would use an audio taper pot.
Click to expand...
This gives the widest range of volume control, possibly overdriving the PI (edit: and/or Power Tubes) at higher settings.

Option #2 would be the top schematic. This retains R14 as R5 (top 470k) and replaces R15 with a 1m (VR4) pot in parallel with a 1m (R7) resistor. The resistor in parallel with the pot gives an overall resistance of 500k ohms. Placing a fixed resistor in parallel with a linear taper pot is a method of creating an "audio taper like" pot when an audio taper pot is not available. (The fixed 1m resistor and 1m pot could be replaced with an audio taper 500k pot with the same result.) This would give a volume range that maxes at the same fixed volume as the original TT B-Flex.
 
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IMG_8789.jpeg

10K gridstopper for V2a aka gainstage before splitload PI stage.

@GAStan cathodyne splitter there so unity gain at most and poor resistance for blocking distortion.

I’d try option 1 first to see what are V2a+b limits with signal level.

Edit. That’s NOT cathodyne phase splitter. Early long tail pair or what the heck? Or is it some iteration of paraphase?
 
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Ok, let's try Option 1 and see if the overdriven V2a+b sound musicals. If not I'll go for option 2.

Ok for the grid stopper on V2a, but why not on V2b also ?
I also notice there are no grid stopper on V1a and V1b. Could this be an good enhancement to add grid stopper here too ? My room is probably not RFI/HFI free..
What kind of resistors should grid stopper be (power wise) - 1/2W ? 1W ? More ?

I repost the schematic and the layout here to avoid switching back to page 1 :
Schematic.jpg
bflex-layout_page-0002.jpg

I have another question, there is an unused 5V supply in the PT. Can I use it to light LEDs inside the enclosure ?
PSU schematic.jpg
 
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0.5-1W is all ok. The amps I have worked with have had 0.25-0.5W grid stoppers even on power tubes. I use bigger wattage ones as they’re sturdier for floating connections and have longer leads.

Btw I looked PI on latest schematic you posted and it’s not split-load PI, but grid stopper is still beneficial at v2a as it makes a long run from previous plate/stage.
 
It may be a paraphrase inverter as I found someone who made some modifications in a that amp years ago mentioning it :

Quote :
"Hi,
I made a bflex for my bass player a few months ago. It was working fine but, even if I added a line out, it was good for practice, but too weak for the rehearsals with our heavy rock Drummer.
So he asked me if I could upgrade it to 100 Watts.
Here's what I did :
- Replaced PWT and OT with your Soldano 100 Watts ones.
- replaced the 2x6V6 by 4x6L6GC
- Replaced the PI Tube with a ECC82
- Added a Hammond 159Q choke.
- Kept the TT PSU mini CT, but added 2x220 MF for B1 (so I have 210 MF for B1)
- Added 2x2 UF5408 before the D1 and D2 of the PSU.
- Added a Standby just before B1, So the 210 MF are charged First.
- Fuses of 4AT AC Input, and 2AT HT
- R6PSU is now 22 K
- Screen resistors = 470 Ohms 5W
- Grid stoppers = 10k
- NFB Resistor = 47 K on the 16 Ohm Tap
- R19 of the Paraphase invertor is now 517 K to minimize the unbalance (still unbalanced though)
- Bias taken on the 60V tap of the PWT, R101 is now 4.7K
- B1=480V, B2=477V, B3=361 V, B4=351 V Bias -53V
- 6L6GC Biased at 40 mA (I installed 1 Ohm resistor on the Cathodes)
-> for 1 Khz sinus input, I got 110 Watt RMS under 8 ohms

The amp so far is working great, very powerfull on 4 ohms with two Ampeg cabinets 1x15."

@JohnnyCreepy : Note that the guy did add grid stoppers of 10k, without saying exactly where, just like you recommend.

EDIT : I have an answer : they use shielded cables between input impedance and V1a, so I suposed a grid stopper resistor can be avoided. About V1b, a grid stopper can alter the tone here (or at least reinforce the mid, which might be a good thing for a bass amp - but I'll start without it and figure out later). So I'll rely on your knowledge and just add one in front of V2a.
 
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XjFred said:
I have another question, there is an unused 5V supply in the PT. Can I use it to light LEDs inside the enclosure
Click to expand...
Didn’t notice this earlier, but if you want to pollute your signal with noise go for it! With careful planning it’s doable, but there’s a reason why chassis lights are rare. Gigging people have problems with stage lights etc. My 100w amp picks ticking noise from slightly worn light ballasts few meters away so I play my rehearsals without lights. :D

XjFred said:
Note that the guy did add grid stoppers of 10k, without saying exactly where, just like you recommend.
Click to expand...
He is talking about power tube stoppers there. Grid stoppers reduce blocking distortion, oscillations and RF interference. Preamp grid stoppers can be anything between couple kilo ohms to hundreds. These grid resistors should be placed right into tube socket grid pin lugs with as short lead as possible on lug side. Most designs get away placing ’em further than socket lug, but it’s a thing to rule out if having some problems on an amplifier with unsteady operation.

I’m getting a copy of 1953 RCAs ”bible” cookbook for AF/RF circuit design soon and gonna quote things from there for sure. It doesn’t cover people gonna be playing square waves through tube amp, but still it’s covering most of things we’re looking here. :)
 
Ok, no lights when I'll start the damn thing. I got it. :LOL:

I think now I have all my answers. Time has come to order.. One last thing : I will opt for a switchable 1Mohm log MV pot to include a bright cap. 100pF seems a good value to start (good effectiveness for bass frequencies ? ceramic disc cap ?)

I really want to thank you all for the time you spent answering my noob questions. And for all the documentation. The LenardAudio website turned out to be very informative.
 
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XjFred said:
Ok, no lights when I'll start the damn thing. I got it. :LOL:

I think now I have all my answers. Time has come to order.. One last thing : I will opt for a switchable 1Mohm log MV pot to include a bright cap. 100pF seems a good value to start (good effectiveness for bass frequencies ? ceramic disc cap ?)

I really want to thank you all for the time you spent answering my noob questions. And for all the documentation. The LenardAudio website turned out to be very informative.
Click to expand...
You have good questions here! Not too hard and all the tube related & time consuming reading sessions finally find use. I’m def a noob too when calculus is involved and still learning with a long road ahead.

I have 3 amps on my bench atm. One is mostly done, but needs time for troubleshooting session about loss of signal. Second is gutted and PCB just got populated, but gonna get remake after I get it together working and decide what to build on it. Third one is partly build, but some parts are not soldered yet, power transformer is unconnected and some wiring is missing.

For all-purpose-try-this-out-caps buy a box of 1-2kV multilayer ceramics from ebay, aliexpress or similar. You get pF to nF range caps to play with for 10-20 coins! When the right value is set, replace it with higher end poly cap (f.ex. Wima) for reducing microphonic effect what ceramics tend to have on circuits prone to pick vibrations.
 
I'm actually going to order a set of ceramic capacitors from Amazon. I'll run some tests.

I couldn't help but look into how to add a pair of 6V6GT in addition to the one already there. It doesn't seem that complicated (as long as the transformers can handle it); it's almost a copy-and-paste of the original power stage.
Maybe that’ll be the next modification in 2027. But for now, let's try to make this thing play music when we need it to and stay quiet the rest of the time.

Good luck with your three ongoing projects!
 
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