How's This Design Looking?

[lkrasner]

Finally got my first custom design done, and I'm hoping for some input from the veterans around here.

First, the tone I'm going for: On "Rhythm" I would like it to be fairly clean, some breakup near the top of the gain range is fine, but I plan to use it largely with effects, so not too much distortion is needed. On "Lead" there should be quite a bit more crunch, but no need to get to full out metal level. looking for more bluesy, up to a more classic rock sound. Overall, I want it to be a fairly versatile classic rock like amp, but I'm not looking for any exact sound.

The main interesting thing is the blended parallel output stage. I've seen this done with some really nice results before, so I thought I'd give it a shot. The idea is to put an EL84 for a more British sound, and a  6V6 for the American tone, with a MV for each to give you full control. I would also like to reserve the option to leave only 1 tube in, possibly with a different octal like a 6L6, EL34, or KT88. This is what the adjustable bias circuit is for, I understand that you usually don't need it with cathode bias.

Now my questions. Of course, any other input is welcome:

Preamp:
1. will the lead / rhythm switch function as expected and give me the previously mentioned tone? If not, where should I look to make changes?
2. I'm probably going to add a fully buffered effects loop after the tone stack. Good idea?

Poweramp:
1. Is the tube blending thing set up in a reasonable way? Will the MV setup I have work, or do I have to do soething to drive each one in a different way? Would it be better to have one MV, and somehow a blend pot?

2. Any OT reccomendations? I would like to have something with a 4, 8, and 16 ohm output since this will be a standalone head.  I assume I probably want a primary of around 4-6K?

Power Supply:
1. Choke filtering a good idea? Someone here recommended it to me, so I'm assuming yes...

2. Are all my resistor/ cap values reasonable?

3. Is it OK to run the final gain stage and cathode follower off the same supply as the screens for the poweramp, or should I add a stage of filtering?

4. I'm just realizing I totally forgot to elevate the filaments in this schematic. I should probably do that with the center tap on the 6.3V winding right?

Any input is definitely welcome. I'm sure I have many mistakes or things I can improve.

Thanks!


EDIT: I'm aware that the component numbering is god awful, I just haven't gone through and redone it yet...

[DummyLoad]

R10 & R13 are too large of a value to be of any practical use and 1/4-1/2W conventional pot there will likely last just a few seconds. R10 & R13 probably need to to be 2-5W pots or rheostats: they would need to be larger rheostats at that, IF you're planning to use 25W+ power valves. a switchable cathode resistor for different types of valves would be be best with a SMALLish 2-5W pot to allow for +/- 10-15% bias R variance.

your PS is going to deliver around 375V - perhaps too hot for EL84s. yes, it'll likely work, but they probably won't last.

for bias parts, another option is adjustable power resistor.

a 6V6 will be near 2x the bias voltage and 2x the drive voltage requirements of the EL84. the 6L6 will be about the same with nearly 2x current of the 6V6 so the cathode resistor would need to be sized for 2x the power handling.

a 3 wire power resistor - adjustable:
http://www.digikey.com/product-detail/en/AVT02506E100R0KE/AVT25-100-ND/257625

5w pot:
http://www.digikey.com/product-detail/en/026T419S101A1A1/026T419S101A1A1-ND/468347

--pete

[lkrasner]

R10 & R13 are too large of a value to be of any practical use and 1/4-1/2W conventional pot there will likely last just a few seconds. R10 & R13 probably need to to be 2-5W pots or rheostats: they would need to be larger rheostats at that, IF you're planning to use 25W+ power valves. a switchable cathode resistor for different types of valves would be be best with a SMALLish 2-5W pot to allow for +/- 10-15% bias R variance.

I've seen mixed opinions about this. Some people have told me that I don't need a large value, others that I do. How should I properly calculate the power across these pots.

The reason I put them in is mostly because the different tubes will change the B+ a fair amount, and require very different bias resistors, especially considering we would be removing the EL84 when using a single bigger tube.

So Let's say this. The configurations I want are: 6V6+EL84, single 6L6GC, and single KT88. how should I properly calculate bias resistor values for these tubes if I were to not use a pot? If I were to use your trimmer idea, what would you define as "Smallish"?

your PS is going to deliver around 375V - perhaps too hot for EL84s. yes, it'll likely work, but they probably won't last.

Once again, a lot of people have told me that EL84s can be run this hot no problem, but let's say I want to be safe. Should I find a different transformer, or should I just use a larger resistance choke or resistor for the first filter stage?

[Glennjeff]

At least 3 Watts through those bias pots. They are a bad idea IMHO.

Use switched 10W resistors.
Look at circuits for standard cathode biased, single ended designs at your B+ to get close to the resistor value,
You'll have one small novel socket for the EL84 so that can be fixed cathode bias.
The octal socket will simply need a SPST to change bias between 6V6 and 6L6.
See Hoffman's circuit diagram page here  http://el34world.com/charts/TubeAmpSchematics.htm

If you really feel the need for some bias adjust, just use switches for high and low, good enough for rock and roll.
OR use three of those variable power resistors Dummyload mentioned at about 20%-30% of the value of the fixed bias resistor of each valve.
(Sum of actual fixed bias resistor + half of variable resistor should equal recommended bias resistor for that valve at that B+)


I'm running 4 X EL84's in Class AB Push/Pull at 350 V in my current project amp and that seems a little bit hot on load line analysis. To get B+ down you could either use a valve rectifier or put the choke immediately after the rectifiers and before the first filter cap.


Where did you get this circuit diagram from?

[lkrasner]

Where did you get this circuit diagram from?

I made it. Poweramp section is all my own design, preamp and power supply largely borrowed from ax84

[Glennjeff]

Hi lkrasner,

I was editing my response while you were replying.
You may choose to re read it.

Great way to master stuff is "just do it", there may be some surprises in store though.

All the best
Glenn

[PRR]

You asked.

There is 200V on plates. You can NOT connect them direct to grids without a coupling cap (in most cases).

When a coupling cap goes "nowhere", as when your Rhythm/Lead switch is in the other position, the coupling cap will charge-up unless you bleed its leakage. That makes a POP when you throw the switch.

Putting the tone stack between driver and power bottles is a sure way to under-drive your power bottles, so what are all this power-bottle frills? Most of these tubes need 25V of drive. This tone-stack has 10:1 loss at many settings. So the driver before it must deliver 250V peak (500V peak-to-peak) of drive to bring the power bottle to full song. It can't do it. The power bottle loafs down in its quarter-power range, and there isn't much difference one to another if you barely tickle them. Also you have a lot of weight and cost for not very much room-filling loudness.

I have no idea what you thought you were doing in your 4th stage. Certainly you do not want to feed your tone stack from ground. I do not think it is wise to have a diode connected that way-- when the grid wants to go negative there is no stopping it. I think you have looked at Merlin but plagiarized very inaccurately. Stealing ideas is not like stealing from banks, you can't leave loot behind unless you really know your business.

The mix-and-match idea does not really work. The parameters must suit each other: power supply voltage, current, load impedance, tube dissipation. If you use a low impedance so the big tubes can flow some current and do good work, then starving current from little tubes so they do not melt will give a thin very-low-power tone. My best attempt was for the "bigger" bottles: you can run EL34 6L6GC KT66 7027 6550 KT88 all at 400V 50mA and 8K SE load. This is 20W Pdiss and the mighty 6550 was loafing at half what it could do, but it happens that with this series you can run the same bias resistor for all of them (and there is a trick for G3 of EL34 and another for 7027). But when you throw 12W bottles like 6V6 in, you have to down-size the whole plan and then the 6550 gets pointless; when you throw high-gain EL84 in then you have to dabble the bias.

Those cathode trims: with "happy" conditions the cathode resistor dissipates about 0.06 of what the plate dissipates. For 12W tubes, 0.72 Watts; more for BIG tubes (if worked at full rating); less for EL84 (because it takes less bias). There's several problems: common pots are not rated for over 0.25 Watts, if you use only part of a pot (such as 300 Ohms, a common size for bias) you only get part of the pot rating, and common pots are now hard to find in 1K value. There are (or were) Mil-spec pots which could be run at 2 Watts and came in 1K, but they are expensive (I paid $13 the last time).

[PRR]

Connections to Bass pot are unconventional. Though it may bend the bass, I don't see the point of doing it this way.

[DummyLoad]

You asked.

There is 200V on plates. You can NOT connect them direct to grids without a coupling cap (in most cases).

When a coupling cap goes "nowhere", as when your Rhythm/Lead switch is in the other position, the coupling cap will charge-up unless you bleed its leakage. That makes a POP when you throw the switch.

Putting the tone stack between driver and power bottles is a sure way to under-drive your power bottles, so what are all this power-bottle frills? Most of these tubes need 25V of drive. This tone-stack has 10:1 loss at many settings. So the driver before it must deliver 250V peak (500V peak-to-peak) of drive to bring the power bottle to full song. It can't do it. The power bottle loafs down in its quarter-power range, and there isn't much difference one to another if you barely tickle them. Also you have a lot of weight and cost for not very much room-filling loudness.

I have no idea what you thought you were doing in your 4th stage. Certainly you do not want to feed your tone stack from ground. I do not think it is wise to have a diode connected that way-- when the grid wants to go negative there is no stopping it. I think you have looked at Merlin but plagiarized very inaccurately. Stealing ideas is not like stealing from banks, you can't leave loot behind unless you really know your business.

The mix-and-match idea does not really work. The parameters must suit each other: power supply voltage, current, load impedance, tube dissipation. If you use a low impedance so the big tubes can flow some current and do good work, then starving current from little tubes so they do not melt will give a thin very-low-power tone. My best attempt was for the "bigger" bottles: you can run EL34 6L6GC KT66 7027 6550 KT88 all at 400V 50mA and 8K SE load. This is 20W Pdiss and the mighty 6550 was loafing at half what it could do, but it happens that with this series you can run the same bias resistor for all of them (and there is a trick for G3 of EL34 and another for 7027). But when you throw 12W bottles like 6V6 in, you have to down-size the whole plan and then the 6550 gets pointless; when you throw high-gain EL84 in then you have to dabble the bias.

Those cathode trims: with "happy" conditions the cathode resistor dissipates about 0.06 of what the plate dissipates. For 12W tubes, 0.72 Watts; more for BIG tubes (if worked at full rating); less for EL84 (because it takes less bias). There's several problems: common pots are not rated for over 0.25 Watts, if you use only part of a pot (such as 300 Ohms, a common size for bias) you only get part of the pot rating, and common pots are now hard to find in 1K value. There are (or were) Mil-spec pots which could be run at 2 Watts and came in 1K, but they are expensive (I paid $13 the last time).

cathode follower will work with direct coupling. 100K plate to grid of cath. follower should be 100R. 100K from grid of CF to bootstrap diode should be 10K and not 100K.
http://valvewizard.co.uk/dccf.html

missed that wonky tone stack wiring.

it is going to POP LOUDLY when you "channel" switch.

consider switching out the 2nd stage from the 1st stage to the 3rd stage, instead of the 2nd to the CF. that way the CF is direct coupled permanently.

attached is an amp we built several years ago. we're pushing the 6V6's hard. 96mA for both. or about 17W each. they are NOS GE 6V6GTA.

--pete

--pete

[Glennjeff]

Hi lkrasner,

Noting PRR's and Dummyloads responses (they REALLLLYYY know their stuff)

Put the tone stack after S6 SPDT, then redesign V6 section (with maybe a 12AU7, 12AT7 or 12BH7), look at some more "working amp diagrams" as to the way you should provide drive to a single ended tube power section.

A FET source follower after V6 MAY be usefull, see tubenit's older D'Mars diagrams for how to create a FET source follower. ( search forum for D'Mars)   

Don't be discouraged by a few little drawing errors.

Do you know appropriate electrical safety procedures for working around High Tension Power ???

[jojokeo]

My suggestion is keep it simple at first. Tweak design once up and running, then tweak some more cycle and repeat. Don't try to put in too many bells a whistles in preamp because you just want as versatile an amp as possible and same with power tubes. You can't please everybody all the time and you can't please all the tubes all the time either. For low gain input you don't need or want more than two stages or you won't stay "clean". For clean tone less is more. For saturated tone then have at it but you'll find you don't need 10 stages. Power amp saturation is preferred way more than preamp buzz. In the end do what you want to try and gain experience and findings as a result. You'll likely re-build this amp several times before you're through trying many things. It's how you learn and grow so don't over analyze the initial schematic design - just put a sound working plan in so that it at least will work. Then get the build going and make adjustments as you go along. Many times the end is much different than the beginning was intended -  especially when just starting out.

[kagliostro]

A FET source follower after V6 MAY be usefull

V6 is arranged as CF so, no need for a SF after V6

(Assuming as reference the schematic linked on the first post)



---

A friend built this and is happy with it (I mean the part of the circuit breaker + CF) (does not use the switch while playing)



K

[Glennjeff]

Hi kagliostro

Good point, but from original, V6 cathode is grounded so will not work. I was thinking use standard plate loaded V6 then a cathode/source follower.

You show a "successful" design using  a tone stack, after cathode follower, directly driving the power tube so maybe lkrasner could use that idea as it is more in line with original concept. 

All the best
Glenn

[jojokeo]

Good schem Kag. Now use input V1a as the Lead IN and go directly to V1b as the Clean IN with a Gain control inbetween and/or switched here rather than like before and you have something.

[kagliostro]

@ Glennjeff

Yes, you are right the CF on the schematic is missing the cathode resistor (I assumed it as a mistake on the drawing)

about the TS preceding the power tubes, this didn't change the fact that there is a considerable loss of signal

all is to say what make you happy, the result satisfied my friend, may be other are not satisfied the same way

K

[kagliostro]

@ Jojokeo

Something like this ?



Franco

[terminalgs]

On the subject of mixed output tube types that require different biases: this is not really a good design unless you pick a single bias resistor that works for both.  This generally means picking one that is sub-optimal for one tube type (or certain death for the other).  Switches or pots that select bias voltage are a bad idea on any other than a test-mule. Why?  they will eventually get switched to the wrong setting for the wrong tube, and destroy a tube (or worse).  Even if you are building the amp and saying "I'm the only one that will ever us it" (which I don't why anyone would say that..)...  in time, some day,  you'll flip that switch! (I consider myself to be a careful person and I've done it!).

my advice for designing a dual purpose lead/rhythm or british/american hybrid is to do all that selection in the pre-amp (like switch bypass caps on V1.  Start with Either 10uf and 25uf, or 10uf and nothing...)  I'd build a solid power amp section with two 6L6's (which, if you are using two output tubes and 1 transformer, for a few more dollars, you can built a push-pull amp and get all the benefits of that design.

[kagliostro]

if you are using two output tubes and 1 transformer, for a few more dollars, you can built a push-pull amp and get all the benefits of that design

I'm prone to think

if you are using two output tubes and 1 transformer, for a few less dollars, you can built a push-pull amp and get all the benefits of that design

K

[lkrasner]

I'm inclined to keep the SE design because I like the looser sound.

I'm also inclined to keep the dual tubes, because it is unique, and adds something interesting to the amp. This is a guitar amp.  I see no need to do things "right" as long as it will work, last well, and produce a good sound.

This idea has been done before. The Bogner Memphisto sort of does it. It has separate preamps, but everything comes into 1 OT in the end.
http://www.bogneramplification.com/mephisto

The Egnater Rebel does exactly what I'm going for.
http://www.egnateramps.com/EgnaterProducts/Rebel/Rebel30/Rebel30Head.html

Here's a couple AX84 threads on the topic:
http://ax84.com/bbs/dm.php?thread=298616
http://ax84.com/bbs/dm.php?thread=284057

Anyway, I've updated the schematic. I fixed the tone stack and cathode follower, those were just stupid mistakes. My computer crashed while making it once, and I quickly redid it without checking everything. I will clean up the reference numbers when I have more time. I removed the old lead / rhythm switch and moved it to the input, switching the first valve in and out. I also removed the adjustable bias.  I can figure out values later, How's it looking now? I'm still somewhat concerned about the high B+ for the EL84. What would you recommend I do here?

Even though I've kind of been ripped to shreds, I'm trying to learn here, and I'm definitely succeeding at that. Thanks for all the help

[kagliostro]

I've nothing against SE, only OT is more expensive and you have more humm

---

In your latest schematic you swap the input but didn't disconnect the circuit from the previous gain stage

when your input is at V1/b you must disconnect the grid of V1/b from the previous gain stage (disconnect the wiper of the gain stage)

K

[lkrasner]

I've nothing against SE, only OT is more expensive and you have more humm

---

In your latest schematic you swap the input but didn't disconnect the circuit from the previous gain stage

when your input is at V1/b you must disconnect the grid of V1/b from the previous gain stage (disconnect the wiper of the gain stage)

K

How's this? changed the switch to a DPDT (shown split in the schematic).

[kagliostro]

Something like this (note that I don't know if this will pop if switched while playing)



K

[lkrasner]

Something like this (note that I don't know if this will pop if switched while playing)



K

That makes sense... I know what I'm doing...

[Glennjeff]

Hi lkrasner

I have a PP amp that can use 2 EL84's and 2 6V6's at the same time and they work (sound) very well together so stick with your idea.

I mentioned some ways you could drop B+ in an earlier comment.

That revised circuit looks like it would work to me, although I'm not sure what D3 is for, or whether a more conventional V6 cathode follower layout would serve better.
EDIT: Oh, I see here http://www.valvewizard.co.uk/dccf.html what you are trying out. Recheck your resistor values around V6.

All the best

[jojokeo]

Even though I've kind of been ripped to shreds, I'm trying to learn here, and I'm definitely succeeding at that. Thanks for all the help

To be sure - everyone is trying to help and assist you, not "ripping you to shreds". You likely don't mean it to sound that way but simply don't get too sensitive. It's okay, everyone's here to help, learn, and share experiences. BTW - aside from all this schematic talk & planning - how's your building skillz? It can all be for naught if you're not keen on this aspect of building no matter how much planning you do.

[lkrasner]

Ok, let's talk effects loop, because that is going to affect my changes. I think I want to add a buffered effects loop after the preamp. Perhaps this: http://i.imgur.com/NL44syH.gif

I originally had the tone stack after the cathode follower thinking it would drive the stack better. So question 1 is this: should I (A) leave the tone stack where it is, or (B) put it between stage 1 and 2.

Assuming A: how should  Incorporate the effects loop. I feel it should really be driven directly from a cathode follower, so should I add a second one? Where would I split the signal for the bypass? After the new cathode follower, right at the FX jack I assume?

B: If I move the tone stack, I already have the cathode follower unused. I assume I could drop this circuit in there. Could I use switching jacks to provide a bypass when unplugged?

Thanks,

Luke.

[lkrasner]

Even though I've kind of been ripped to shreds, I'm trying to learn here, and I'm definitely succeeding at that. Thanks for all the help

To be sure - everyone is trying to help and assist you, not "ripping you to shreds". You likely don't mean it to sound that way but simply don't get too sensitive. It's okay, everyone's here to help, learn, and share experiences.

Absolutely understood, and I did not mean to sound snarky or sensitive. I would have been dumb to think everyone would praise my design and call me the next Dumble. I woke up this morning and actually laughed when I saw how many problems people had pointed out to me over night. That is encouraging to me. It means the "pros" here are actually looking at my work, not just glancing at it and saying "yeah, that's fine".

BTW - aside from all this schematic talk & planning - how's your building skillz? It can all be for naught if you're not keen on this aspect of building no matter how much planning you do.

My building skills are very good. I've done plenty of other electronics projects which got my soldering skills up, and I have plenty of experience around high voltage stuff like in guitar amps. My first amp build was a success, and went very smoothly, with just a few issues revolving around improper measurement for chassis holes.

here she is, a slightly modified AX84 P1EX. AWESOME sounding amp by the way for anyone that's considered one. Sorry for the bad picture:

[jojokeo]

Great to hear and the amp looks good - from the outside  ;)  (most here like to see gutshots)

[lkrasner]

Great to hear and the amp looks good - from the outside  ;)  (most here like to see gutshots)

Yeah, well this is the pretty part :p

I need to redo a bit of the wiring inside to really make it look nice, then I'll post a gut shot. It's not bad, just my wires from sockets to the turret board are pretty messy.

[jojokeo]

 

[kagliostro]

I feel it should really be driven directly from a cathode follower, so should I add a second one?

Tubenit has done something similar on his amp, I don't remember if Tweed Overdrive Special II or D'Mars or which other

EDIT: On the D'Mars there is

http://el34world.com/Forum/index.php?action=dlattach;topic=15406.0;attach=36175

K

p.s.:You must install JSChem to read the .sch files

http://dhost.info/jschem/

[lkrasner]

I feel it should really be driven directly from a cathode follower, so should I add a second one?

Tubenit has done something similar on his amp, I don't remember if Tweed Overdrive Special II or D'Mars or which other

EDIT: On the D'Mars there is

http://el34world.com/Forum/index.php?action=dlattach;topic=15406.0;attach=36175

K

p.s.:You must install JSChem to read the .sch files

http://dhost.info/jschem/

Interesting. That is more or less what I was thinking, but I'm cunfused about the bypass. It still goes through both stages. Isn't this a problem?

Also, assuming I move the tone stack, I will have an extra half 12ax7 that is unused. Should I do something about that? Since  I have the 2 tube thing, could I somehow have 2 recovery stages fed by the same input, but with the outputs split to each MV? Perhaps that would improve the response of the poweramp.

[lkrasner]

Updated again. How's this looking? I added an FX loop.

I'm thinking I should probably add another stage to the power supply. Power tubes of B+1, screens and FX off B+2, Tone stack CF and preamp stage 3 off B+3, and preamp stages 1 and 2 off B+4. Does that make sense?

[kagliostro]

To me the 220k preceding the grid of V2 must be moved before the switch, connected to the wiper of the gain pot

or you have a high loss of signal when the input is directed to V2

look to my example

K

[Glennjeff]

R26 should be 100 ohms not 100k methinks.

[lkrasner]

R26 should be 100 ohms not 100k methinks.

You are correct. Fixed.

[terminalgs]

some observations:


R21 and R22 are a voltage divider.  its ok when S5 is in the high-gain position, but undesirable in the low-gain position. the difference between the two positions in terms of signal fed to V3 could be 0.25Vrms vs 20Vrms (or more).  If you are trying to limit that big signal when S5 is switched to high gain,   remove R21,  and put a resistor between C8 and R18, maybe 470K.


I'm a big believer in grid-stoppers.  the only ones I see here are R2 R5 and R38.  I'd add grid stoppers to V4, V3, and V5. Some would even add them to V6 and V7. 


If S5 gives an audible pop, you can change S5A:  remove it from it current location, and let your input signal go to the grid of V4 regardless of switch position.   I'd do this anyhow, there is no benefit from having it switch signal on the grid.


you have do many signal voltage dividers (R21/R22), (R25,R24), *big one* (R26,R27) that if you reconsider all these dividers values,   you might be able to remove a gain stage.  instead of Gain>Divide>Gain>Divide>Gain>Divide  you might just be able to simply "Gain>Divide>Gain".

[lkrasner]

Updated again. I fixed the 100K resistor and fixed the 220K divider per kagliostro's recommendation. I may add grid stoppers later if I find I need them, The should probably be put right on the socket anyway, so it won't seriously affect the layout.

I also renumbered everything to make sense.

I think I'm ready to tackle the layout now, but I want 1 last check. In terms of layout, should I keep everything on one board, with the power supply caps spaced along it kind of near the stages they serve (AX84 style), or should I use a separate cap board like many of the early Fenders? Or possibly a poweramp and preamp board? What are the pros and cons of these options?

[kagliostro]

It will be nice if you post your new schem (so we can see how it is now)

---

To me, if you want the first filter cap can be placed in a separated PS board

about the other filter cap I would like to dispose it near the part of the circuit they feed

see my 7-9 cylinder layout



K

[lkrasner]

Quick question : how much current should I expect the preamp on this to draw. I know 12ax7s draw about 1mA per triode, but the 12au7 is rated to 10mA should I actually expect it to draw this much? Is it different when set up as a cathode follower?

[lkrasner]

Alright, so I built this thing!!

Of course, there are some issues. At first full startup, it was very low volume and super distorted. Very not good. checked the voltage on the plate of the FX loop recovery triode and it was dropping down to 111 volts from a supply of 290ish. Very not right. I swapped in a 12ax7 for the hell of it, and it made it a bit better, but still not right. At this point, I simply bypassed the whole FX stage, running the wiper of the treble pot right into the volumes. That fixed it. With the gain switch on Dirty, the tone was pretty darn good. the tube blending works spectacularly. It's nice and smooth on the 6v6 alone, adding the el84 in brings about a very noticeable tone difference. Gain goes from clean to AC/DC style crunch pretty well.

The clean setting isn't working as well. The volume is quite low, definitely way less than when It's on lead and still kept pretty clean. It also just sounds kind of meh. not very crisp or clear somehow. This leads into the final issue...

The tone stack is not right. the  treble pot has a HUGE effect, swinging from super bassy and cold to super bright treble. The bass pot has almost no effect at all regardless of the position of the treble pot. I've checked that my wiring is exactly as I have it in the layout (see below).


Here's the latest schematic and layout. Note that in the layout, the switches and jacks may seem off, but they are right in the actual amp. It is also a bit messy, I was a bit rushed in designing it. I also left out the fuse on the center tap of the power transformer.

[lkrasner]

Alright, so I built this thing!!

Of course, there are some issues. At first full startup, it was very low volume and super distorted. Very not good. checked the voltage on the plate of the FX loop recovery triode and it was dropping down to 111 volts from a supply of 290ish. Very not right. I swapped in a 12ax7 for the hell of it, and it made it a bit better, but still not right. At this point, I simply bypassed the whole FX stage, running the wiper of the treble pot right into the volumes. That fixed it. With the gain switch on Dirty, the tone was pretty darn good. the tube blending works spectacularly. It's nice and smooth on the 6v6 alone, adding the el84 in brings about a very noticeable tone difference. Gain goes from clean to AC/DC style crunch pretty well.

The clean setting isn't working as well. The volume is quite low, definitely way less than when It's on lead and still kept pretty clean. It also just sounds kind of meh. not very crisp or clear somehow. This leads into the final issue...

The tone stack is not right. the  treble pot has a HUGE effect, swinging from super bassy and cold to super bright treble. The bass pot has almost no effect at all regardless of the position of the treble pot. I've checked that my wiring is exactly as I have it in the layout (see below).


Here's the latest schematic and layout. Note that in the layout, the switches and jacks may seem off, but they are right in the actual amp. It is also a bit messy, I was a bit rushed in designing it. I also left out the fuse on the center tap of the power transformer.

Got everything working but the tone stack. Any help would be greatly appreciated.

[Willabe]

I looked at both drawings, side by side and I ~think their correct.

Go over all the parts in the TS to make sure their the correct values. Like a 10K resistor instead of a 100K. Or a wire that's broken inside it's insulation that you can't see the break.


                  Brad   

[lkrasner]

I looked at both drawings, side by side and I ~think their correct.

Go over all the parts in the TS to make sure their the correct values. Like a 10K resistor instead of a 100K. Or a wire that's broken inside it's insulation that you can't see the break.


                  Brad   

I've checked the layout/ actual build / schematic a million times too, so I'm pretty darn sure it's correct. I'll go carefully check all the wiring and see what I can find.

[sluckey]

The tone stack is not right. the  treble pot has a HUGE effect, swinging from super bassy and cold to super bright treble.

Use a 250K log pot for the treble pot (like Fender or Marshall). Does that help?

[lkrasner]

The tone stack is not right. the  treble pot has a HUGE effect, swinging from super bassy and cold to super bright treble.

Use a 250K log pot for the treble pot (like Fender or Marshall). Does that help?

That fixed the treble pot. also put a 500k log pot in for the bass, but the bass knob still appears to do nothing.