Hybrid Tweed Tremolux/Brown Deluxe

[tubeswell]

Hiya everybody

After building a 5E3 a couple of months back, I decided I'd like more drive and power in the 5G9 I built, so I was wondering about the possibilities of adding more drive (while I impatiently wait for the last part for my 6G15 build to arrive).

So seeing as how the number of tubes and B+ voltage are somewhat similar to the 6G3 (i.e.; 2 x 6V6Gt fixed bias, plus 3 pre-amp tubes with a tremolo effect, with about 374VDC B+), I thought it'd be a cinch to make up a new eyelet board and whack it into the 5G9 chassis I built with a bit of re-wiring. So I came up with this 5G9/6G3 blendy thing (albeit with the traditional narrowpanel deluxe vol/vol/tone setup). I'm pretty sure the voltages would be about what I have guessed, because I get a 374VDC B+ in the 5G9 with a 5U4G recto and 40uF reservoir cap going into a choke/capacitor input filter. Whaddaya reckon? Any fatal flaws that you can see? (Shoot me down if you must)

[txstrat]

Hi TW
Only a typing error, maybe. In the B+ line right after the choke theres a resistor marked as 10w. Shouldn't that be 10k 2w?
What are the diodes before the rectifier tube for?
Great drawing.

Have you already measured all the voltages? In my build I tried -26v bias voltage (like in the 6G3 schematic) and came up with a little short of 40ma (1 ohm resistors measured). I decided to go for -28v then.

Matt

[pdf64]

Nicely done schematic - I guess those diodes are there to protect the PT from a shorted 5U4?
The anode circuit of the 12AX7 after the tone/vol controls looks like you've missed a cap out after a B+ dropper resistor. Peter.

[tubeswell]

Hi TW
"Only a typing error, maybe. In the B+ line right after the choke theres a resistor marked as 10w. Shouldn't that be 10k 2w?"

Ooops! I'll fix that up Cheers


"What are the diodes before the rectifier tube for?"

Protect the Ht winding from a possible rectifier tube short. (In one of R.G.'s immortal amplifier articles)

"Great drawing."

Thanks

"Have you already measured all the voltages? In my build I tried -26v bias voltage (like in the 6G3 schematic) and came up with a little short of 40ma (1 ohm resistors measured). I decided to go for -28v then."

Matt

I've measured the voltages in my 5G9 - and after setting the bias at 25mA, and the grid was about -28V. But when I first built the 5G9, I had set the 50k pot-39k series ground leg of the bias voltage divider at 56k - as it had been in the original schematic - and the grid voltage had initially been -22V (31.5mA), and it blew a fuse the first time I switched the stand-by off and back on again. (So then I adjusted it to about 25mA, but have since turned it up to 28.5mA). So I kind of figure that with the o.a. B+ voltage being the same in both amps' schematics, that the quiescent voltages will most likely be similar to the 6G3 from the B+ point onwards, if I keep the filter choke and screen grid setup from the 5G9, but have everything else in the power supply and plate resistors as per the 6G3. - I realise that with a 5U4G rectifier there'll be a bit more sag than with a 5AR4. However the choke will hardly drop any voltage, and I'm wondering how this will affect the PI and pre-amp under load? (I presume that in the 6G3, the effect of the 1k R/C filter is to make the PI and pre-amp voltages a tiny bit more saggier?)

[tubeswell]

"Nicely done schematic - I guess those diodes are there to protect the PT from a shorted 5U4?"

Yep. (Having said that I realise that there are other 'protections' that could be done)

"The anode circuit of the 12AX7 after the tone/vol controls looks like you've missed a cap out after a B+ dropper resistor." Peter.

Just the confusing way it's been drawn - that coupling cap is a .022 and it is right next to the 100k plate resistor for that stage.

Do you think the tweed deluxe vol-tone configuration will sit alright in the rest of the 6G3 pre-amp? (There is no equivalent to the 6G3's 220k series resistors leading to the next stage - I wonder if those resistors in the 6G3 are a frequency (low-pass?) filter to knock out some of the hiss from the 'higher-gain' 220k plate resistors in the first stage? - In which case would I be better to retain a 220k series resistor before the grid of the 2nd stage??)

[tubeswell]

Fixing errors

I spotted an error in the way I'd drawn the plate resistors/signal path for the first stage (how embarrasing!) so here it is fixed.

I have fixed the 10k 2W mistake (Thanks again txstrat)

I have added a possible 220k series resistor in the signal path before the second stage (query whether the 220ks series resistors in each channel before the 2nd stage in the 6G3 schematic are some sort of low-pass filter to counteract the hiss from the first stage's 220k plate resistors??). Curious to see whether there are any opinions about that (would it actually be better as 220k grid stopper on pin 2?)

Also more info on the PT. I recall it has a 3A heater winding, so I could possibly run a 5AR4 and 2 x 6L6.

[txstrat]

I believe the 220k are just mixing resistors to join the two signal lines before the next grid. To make sure no signal runs back to the other preamp stage. In your preamp that wouldn't be necessary cause you only have one tone pot for both channels with a single line to the next grid. I don't think it would alter the tone much if you put one in, though. It might damp down the signal as whole a little.

And know what? I thought of a different OT and two 6L6 as well, since the headroom don't increase with the vol pot over 5. It just adds more grain to the sound. Not bad, though.

[txstrat]

After having a look at the "new" schematic I found after the 10k 2w resistor in the B+ line you got two filter caps where in between I miss another resistor to drop the voltage. In the 6G3 schematic it is a 27k.

[tubeswell]

I believe the 220k are just mixing resistors to join the two signal lines before the next grid. To make sure no signal runs back to the other preamp stage. In your preamp that wouldn't be necessary cause you only have one tone pot for both channels with a single line to the next grid. I don't think it would alter the tone much if you put one in, though. It might damp down the signal as whole a little.

And know what? I thought of a different OT and two 6L6 as well, since the headroom don't increase with the vol pot over 5. It just adds more grain to the sound. Not bad, though.

Okay I might leave the 220k out as a possible later add in if I need to quell the highs a bit (mind you with the NFB, I don't think excessive highs will be an issue).

Incidentally, the OT that's in the 5G9 is rated at about 25-30W, so I wouldn't need to change it to run the 6L6s.

[tubeswell]

More errors fixed

I took that extra 220k (before the 2nd stage) out and have put the 27k in the pre-amp supply (thanks yet again txstrat)

[txstrat]

I'm quite curious how this amp sounds.

[tubeswell]

Hi txstrat

I'm kind of guessing it'd be like the 5G9 but a bit thicker and with more harmonic distortion as a result of the extra driver stage, and hopefully not to many chirps and peeps from the tremolo when the tone control is at max treble

You got any sound bytes of your 6G3 yet?

[txstrat]

And hopefully not too much hum

No soundfiles, yet. Think on saturday i'll have the time...

[Bruce / Mission Amps]

Hi txstrat

I'm kind of guessing it'd be like the 5G9 but a bit thicker and with more harmonic distortion as a result of the extra driver stage, and hopefully not to many chirps and peeps from the tremolo when the tone control is at max treble

You got any sound bytes of your 6G3 yet?

My old Bluestone amps were an amp very similarly built like this but with 6L6s so I'll tell you... and well, to be honest, set up like this, it maybe fun at first but you probably aren't going to be in love with what you built.
You'll have wickedly too much preamp gain (as drawn) and the phase inverter will clip before the power tubes do.
It will be a nasty clipped sound as you turn the volume control up past room level.
Go re-look at the 6G3 and maybe you'll see where you missed two things and a VERY critical important feature in the preamp that limits this.
If you can't see it and somebody blurts it out, not giving you time to figure it out on your own, I'll be back later to show you...

[tubeswell]

Thanks Bruce

the things I've left out that I'm aware of (but perhaps not aware of the effects of as much) are:

1) The 6G3 vol-tone setup on each channel (which on their own are 'back-to-front' from what you have in a 5E3 or 5F2A tone-vol setup - does that make a difference?, apart from the relative interactivity of the channels in the latter two? - Actually I can see how putting the vol contol first and the tone control last enables one to put a 220k series resistor in after each tone control without it affecting the workability of the 6G3 vol and tone controls) - But... I was wanting to go for a 5E3 style setup for ease of use of fitting it into an existing 5G9 style chassis, so my next hunch is;

2) the 220k series resistors in the signal path after each vol-tone stack. (In Revision 2, I had wondered about this and I had put a single 220k in series before the grid of the 2nd stage as a possibility, but was questioning its purpose - I initially wondered whether it was some sort of low-pass filter to counteract the hiss from the higher gain from the (higher-than-usual) 220k plate resistors in the first stage, but came to the verdict that they were mixing resistors (as txstrat suggested), so were possibily unnecessary with a 5E3 style vol-tone-vol setup. Do they have a dual function as mixing resistors and LPF (and therefore should I put one back in at the grid of the second stage - I presume I could only put a single 220k (grid stopper?) resistor in after the junction of the two vol 'input lugs' otherwise it would stuff up the interactivity of the two vol controls)?

3) The configuration of the first stage having 220k plate resistors going to a common 1k5 cathode resistor bypassed by a 22uF cap. Should I stick to 5G9 values (100k and 820R? and a 12AY7)

4) Or is is something else? (the resistor values in the PI?)