Hybrid Tweed Tremolux/Brown Deluxe

[txstrat]

Well concluded TW.

I'm curious too, if my assumptions were correct..... or not.

[tubeswell]

So further to my above thoughts (reflecting on Bruce's hints) I have modded the scheme Rev 3 for an option (Rev. 3a) with a 220k before the second stage grid. Bruce, if Rev.3a addresses the hints in your previous commentary, then please tell me and I'll stick with that. :-)

However if that doesn't get there, then I can side-track everyone with another 'revision' with three options:

Rev. 4 - going to a 12AY7 (- with 5G9 plate & cathode resistor values) in V1

Rev. 4.1a - with 12AY7 in V1 (as above) and a 220k in series before the second stage grid, and changing the PI values to 5G9 ones

Rev. 4.1b - same as 4.1a but without the 220k

What I want is a thicker sound than the 5G9, a bit more Harmonic Distortion and more drive that happens (sort-of) gradually as you wind it up, and without brutally hard clipping or mosquitoish sounding buzz, or flabby-sounding mush

[txstrat]

TW on the second stage the coupling cap has to go from in between the 100k and the 15k resistors to the PI. I think thats what Bruce meant. When putting it right to the plate there will be too much gain. Please don't ask me to explain why

[tubeswell]

Hi txstrat - I don't think where I've drawn the coupling cap in the schematic would make any difference (except for maybe most people not being able to spot it at first glance ).

However, I do think it's probably more to do with the signal voltages and the padding in between the 1st and 2nd stage that Bruce was hinting at. However, whether a series 220k before the grid of the 2nd stage will provide all or some of the answer I don't know for sure (but I must say that fiddling with schematics is definitely the way to work things out and provoke thought and feedback before fiddling with the actual circuit).

I hope I can get away with a 5E3/5G9 style vol-tone-vol setup - but now I am begining to wonder whether that means using a 12AY7 in V1? (as per Rev. 4), but even with Rev. 4, I'm not sure about which LTP values would be better. Would I get what I'm after with more front-end gain and less PI gain, or the other way around? Hence I have done options 4.1a and 4.1b for comment.

(To further confound things, I guess I could go to a 6G2 style 1st and second stage with the vol and tone controls in the same order as the 6G3, but I would lose the interactivity and tonal options of that 2nd channel - and that would also mean more inter-stage attenuation because of the gainy structure of it all going into an LTP - so I'm not going to go there - for now)

But as an afterthought, I wonder whether moving the NFB to the cathode of that second stage (rather than at the PI) would help dumb things down a bit? or should the NFB stay at the PI cathode?

[txstrat]

TW
I'm quite sure you can leave most things as they are.
A couple month ago I built a Deluxe Reverb II clone and had a bad oscillating problem in the gain channel when turning it fully up. It turned out that I used a 100k plate resistor instead of a split load of 82k and 22k. (have a look at this thread: http://www.music-electronics-forum.c...ead.php?t=8672 )
As a matter of fact I changed the plate load and the amp sounded great at all pots fully up (not even a slight bit of oscillating). I think it's a matter of attenuation and that the signal has to go through a 100k (in your amp) before hitting the next stage. That quite a difference I think.
Hopefully Bruce can help us out a little more.

Anyway, isn't it great to tax our brains to find out??

[tubeswell]

Taxing our brains - you bet bro!

Interesting what you said about the oscillation caused by the plate load resistors. I think I remember reading somewhere that you have to change the tail resistor if you're going to run 2 x 100k plate load resistors in an LTP.

[txstrat]

I think you got to see the plate load resistance as a whole and the "tap" for the coupling cap in between as sort of a wiper on a pot. It then depends on the place where (in between) you acatually locate the "tap" (values of both resistors) to let more or less signal (gain) get through. Would be interresting if a pot could be set here for ajusting gain.

[tubeswell]

I think you got to see the plate load resistance as a whole and the "tap" for the coupling cap in between as sort of a wiper on a pot. It then depends on the place where (in between) you acatually locate the "tap" (values of both resistors) to let more or less signal (gain) get through. Would be interresting if a pot could be set here for ajusting gain.

A pot with DC on it would be real scratchy. There a blurb on designing LTPs on the valve wizard site (along with lots of blurbs for designing all sorts of stages). The attached (word .doc) thread also has really useful discussion about it. (Oops - I've just seen what you were trying to say about the plate load - Gee said the same thing - Shite I must be truly mule-headed dense!)

[Gee]

I built a 6G3 with a single channel and cathode bias and I also failed to register the importance of what Bruce is alluding to - until I fired it up. As a result there was way too much gain!

The way that the split load plate resistor is drawn in the Fender schematic makes it easy to miss.

Below is my schematic that illustrates the measures taken to control the gain.

[tubeswell]

Yet more errors fixed/changes made

Thanks Gee - I could see the split load plate resistor drawn that way on the schematic, but duh, when it comes to drawing software!! (I also fixed a drawing error in the way I had the tonestack shown) Software stupidity now (somewhat) mitigated! I've made a mental note of the way you fiddled with your pre-amp plate resistors/supply - much obliged for your info - I see now that txstrat tried to warn me about this but I couldn't see it at the time - my apologies to txstrat

Summary of gain-management measures in Rev. 3b include:

1) possibly going to 12AY7 in V1, with 5E3/5G9 plate and cathode resistor values, or staying with 12AX7 and 6G3 values (or sliding somewhere in between). Not sure about whether I need the 2nF plate bypass cap (easy come-easy go). Either way I need to watch the gain before the PI

2) the 100-220k 'grid stopper' series resistor at 2nd stage (not strictly a gain reduction measure - more of a hi-freq inhibitor/LFP), but I'm still not sure about whether this is important

3) Properly drawn plate load/supply setup for 2nd stage

4) coupling cap to PI might be tweeked - I moved its' location on the schematic to avoid further controversy :-)

5) fiddling with resistors in the PI. - Not quite sure why the cap at the grid of the 'lower' triode is lifted from the ground in the 6G3, but not in the 5G9 (I guess its to do with having an NFB going to the tail of the PI, and the lower value of the resistor in the tail on the 6G3 schem compared to the 5G9. The voltage at the tail/cathode/biasing resistor node in my 5G9 is +27V, and I note that the 6G3 schematic has 18V here - I guess that's not too critical, but has something to do with the NFB. Nevertheless I have included a range of resistor values in the PI. If I dump the NFB altogether, I take it I can go to something like 22k-39k in the tail with a 470-680 biasing resistor, as per Bruce's suggestion in the thread on LTPs I posted before?)

6) fiddling with resistor in the NFB

On this revision, I have hi-lighted possible tweek values in blue (I can see this is going to be a tweeking amp).

Also yesterday afternoon I went through and measured the voltages in the output stage again my 5G9 with the Sovtek 5U4G, and those values are now shown on the schematic, bar the values for the PI (which I am guessing could vary quite a bit from tweeking with the cathode bias/tail). The voltages with the 5U4G lead me to think that I could get decent voltages for 6L6s if I used a 5AR4, however should I be worried about the 500V filter caps?

I also added in the .01-.05uF 'bleeder' cap at the depth pot wiper, as I can impagine that this was included in the 6G3 for combating undesireable chirps.

Any comments welcome

[txstrat]

Gee,

when does your amp start to break up?
I don't know if you followed my thread for the 6G3 (http://music-electronics-forum.com/s...ead.php?t=9928) but as I mentioned over there my amp breaks up from 3 on the dial. I think that's quite early.

Thanks

Matt

[Gee]

Matt, I have since rewired that amp into something else. From memory it started to breakup about halfway. However, the beauty of the split load is that you can fine tune the excess of gain and tune it down to exactly where you want it by altering the ratio of the 2 resistors forming the split load.

From memory the original 6G3 schematic has a 50% gain reduction where the 2 channels are summed together with the 220k pair and then a further reduction of about 13% with the 100k and 15k split load.

[Gee]

2) the 100-220k 'grid stopper' series resistor at 2nd stage (not strictly a gain reduction measure - more of a hi-freq inhibitor/LFP), but I'm still not sure about whether this is important

I don't think you need that resistor. It is normal to have 1.5K grid stoppers on the 6V6 tubes.

3) Properly drawn plate load/supply setup for 2nd stage

Looks correct now.

[Bruce / Mission Amps]

Well concluded TW.

I'm curious too, if my assumptions were correct..... or not.

Plate voltages are important so try to keep them around 110% to 115% of the original schematic.
The LTP phase inverter has quite a bit of gain for this amp and the NFB loop is not very powerful.
The triode after the volume control is running flat out with no attenuation after it .. so yes, the trick is to use a split load resistor setup on that second triode stage and tame down the third gain stage which is the actual phase inverter.
For that split load stage, you'll probably find that something like a 22K and 75K to 82K resistor works well but, a 100K pot works the best as you can dial it in. Just remember that it works right with the larger resistance value on the plate side of the triode and the smaller resistance value on the B+ side.

Use any 1/2 watt trim pot's wiper as the coupling cap connection point and the outside legs of the trim pot as the actual 100K plate load resistance.
Works great and if a little noisy, so what?
Many of us who build fresh amps start out this way to determine if a stage or previous stages are blasting through creating all kinds of havoc with the amp.
All you're really doing here is finding where you like the pot set and then shut it off and measure from the wiper to each outside leg to find out what two resistors to use. Replace the pot after you like what you get.

With respect to the LTP PI, what you might like is the starting with the LTP having less gain and a better balance... try an 820 to 1K2 biasing resistor and for the tail resistor use something between 27K and 39K for starters.
Smaller value will create more imbalance and a larger voltage swing out of the phase inverter.
Also be prepared to use something smaller then .022uF cap to drive the PI.

The two 220K resistors on the grid of that recovery triode is actual a good thing to mess with... ground one (that will be the correct way to do this) and you'll notice that the circuit is a voltage divider.
Using two 220K resistors will halve the input signal to the triode.
It's a good spot to try different value resistors.
Everybody seems to be going the right way... keep in mind that the two power tubes only needs enough voltage swing to negate the bias voltage to run that set up at full power.
Depending on the B+ rail and drive levels, the LTP can swing 60vac to 120 vac.

[tubeswell]

Thanks Bruce

I've revised the schematic again (3c).

1) Re: the plate for the 2nd stage, I will do what you suggest but perhpas add a 4k7-15k resistor in series with the 'output' of the 100k trimpot so that I can get more plate resistance dialed in in the 'input' side.

2) Re: the voltage divider to the grid of the 2nd stage - not sure if I should go as low as 220k to ground but maybe I could start with 1M (and then if that's no good 470k etc)

3) Re: the resistors in the PI, I have amended them to your suggested ranges, but maybe will leave the NFB at 56k (or possibly less). I will leave the 1k5 resistor to ground as per the 6G3 schematic

4) Re: the voltages - I will try and get the voltages down, perhaps with an EH 5U4GB I have (which seems to pump out less than the Sovtek 5U4G). My other option would be to drop one of those 20uF in parallel at the filter reservoir. But I don't think a 5Y3GT would have enough lemons. (and then there's aways zeners)

Cheers