HoSo56 amp project...design input sought

[tubenit]

Could this schematic be edited to work????

Tubenit

[Fresh_Start]

Late to the game, but this text keeps me thinking:  "it slopes the tone stack as you turn the Master down."  Coupled with something Peter said in the first video, I wonder if the "Master" is mis-labelled.  Maybe the slope resistor is being changed?  That would affect frequency response and insertion loss of the tone stack assuming TMB.  Combine a variable "Raw" control like Dave Allen's (mid boost/tone stack defeat) with a variable slope resistor and you might get a similar affect.  Also, given Heinz's observations about excessive gain, is it possible that the "Master" is actually dumping gain (or not) before the tone stack?

It's great to watch you all working this out.  Thanks!

BTW Jeff there was a lengthy discussion on the 18Watt forum within the past year or so about one of the London amps and how they got an EF86 preamp stage to sound so sweet.  Might be worth chasing.

Chip

[ACDCG400]

link for that 18 watt discussion please lol! their search function just doesnt work lol. =]

[Fresh_Start]

My bad - I haven't been visiting the 18Watt forum for longer than I thought, plus it was the 65 Amps "London" model I was thinking about.  It looks like every useful post has either been deleted or the attachments (i.e. schematics) have been removed.  There was a lively and interesting discussion on the subject. 

Guess we can't blame Dan Boul too much, although someone will (or already has)reverse engineer(ed) his circuit.

Chip

[Heinz]

I've been meditating over the circuit again and spent some time with duncan's tone stack calculator. Here's a new idea. The 'normal' signal path is a standard Fender tone stack, the 'bump' path is a simple CR-filter with switchable caps and a tiny low-pass filter for the tone. I have investigated many possibilities around the slope resistor, but this seems to be the most likely solution. All other tone stack modifications either do not 'bump' or leave the tone controls effective when 'bumped'.

I'm no longer convinced that my previous master volume design is right. It doesn't make sense anywhere before the PI. I'm not sure if Peter's "it slopes the tone stack" is correct. He also says that the tone stack is disabled when the bump section is on. So how can it be part of the tonestack (which is disabled) and still be working? I did some internet reading on master volumes and decided to try a simple post-PI cross-pot, which many say does sound very natural. We'll see.

Excessive gain was another problem of the first design. The AC15 runs the EF86 output directly into the PI and sounds fine, so that signal level seems to be about right. I have now tried to maintain that level at the PI input by attenuating the signal at the volume pot. The ballpark numbers can be found in the schematic.

I'll make some experiments (hopefully this evening) and post the results.

@tubenit: Jeff, I apologize for not using your schematic. I did not want to re-invent the wheel, but I had trouble with the snap grid and could not draw any straight lines, so I started from scratch.

[tubenit]

Heinz,

Great job on the schematic!  As usual!

Da Geezer,

Can you give a thumbs up or edit Heinz's schematic and then I'll edit a layout to match it.

Thanks, Tubenit

[Heinz]

Jeff, I'd hate to see you waste your time...I'd wait with the layout until the design becomes clearer.
I'll post some results tonight. We'll see if this design is on the right track, or if we'll have to come up with a complete new idea (again)...

[Heinz]

Here are some test results:

• the cross-pot master volume is NOT what we want. It sounds very fizzy when turned down. I'll try a dual pot post-PI master volume next.
• the second triode stage adds too much gain. It should either be omitted or we have to lower the signal level even further. I don't really understand this. The 'bumped' preamp is almost identical to an AC15, which feeds directly into the PI. How can this be controllable on the AC15? It's either off or distorted on my amp. Does anyone have experience with AC15s? If the gain stage is dropped, what could we do with the unused triode?
• I have only prototyped the bump channel so far, it's late and I can't turn the volume too far up, but it sounds pretty good. The bump tone pot works and switching the caps changes gain and bass response just as intended. The cap values are not conclusive, they're probably too large. However, the overall 'bump' design does not seem to be too far off.

It's almost midnight now, so I'll continue my experiments on wednesday. Good night everyone!

[Geezer]

Thanks for the work & update, Heinz!

what could we do with the unused triode?

I know it's hard to do (  ;D ), but it's really OK to leave a triode unused! It's entirely possible that's what 65 did.....

I think EF86 > 12AX7 CF > TS > PI is the way to go....is that the arrangement/circuit you have now?

[tubenit]

How about using a 12DW7 instead of a 12AX7 in the preamp to lower the gain? 

Amps like the D'Lite have four gain stages prior to the LTPI.  So there has got to be a way to attentuate some gain with a 12AX7,IMO.  I am also wondering if the 6BM8 tubes are easier to overdrive than the EL84?

With respect, Tubenit

[bluesbear]

"So there has got to be a way to attentuate some gain with a 12AX7,IMO"

No cathode cap and a large cathode resistor; maybe 10k to start. That would definitely lower the gain. If that's not enough, a voltage divider after the tubes output would also lower the gain. It's all a crap shoot, just fiddling around till it sounds right. It's a pain but the only way I can ever figure how something will sound. Getting it from inside my head to coming out of the speakers is not an easy task for me... but it's what I really love about building these things.
Dave

[Heinz]

I know it's hard to do (   ;D ), but it's really OK to leave a triode unused! It's entirely possible that's what 65 did.....

:'( What a waste.
Of course, we could use the triode for something completely different, e.g. a tremolo...

I think EF86 > 12AX7 CF > TS > PI is the way to go....is that the arrangement/circuit you have now?

Yes, that's the topology. Plus the TS can be switched out of the circuit.

How about using a 12DW7 instead of a 12AX7 in the preamp to lower the gain?

Yes, I'm sure that would work, ANY triode will serve as a CF. I just don't have any...

Amps like the D'Lite have four gain stages prior to the LTPI.  So there has got to be a way to attentuate some gain with a 12AX7,IMO.  I am also wondering if the 6BM8 tubes are easier to overdrive than the EL84?

The D'lite and other high gain amps attenuate the signal after each gain stage with a voltage divider. We only have ONE gain stage...maybe we could attenuate the signal before the CF, as Chip suggested some posts ago. Some more tinkering is required here.
The 6BM8 needs a higher drive voltage than the EL84. I'm sure they are not part of the problem.

No cathode cap and a large cathode resistor; maybe 10k to start

I tried that. It definitely reduces voltage gain, but it also reduces voltage swing. Large signals distort easily, which is why e.g. Marshall and Soldano add these stages. You can only keep these clean at low signal levels, which brings us back to our problem. Leaving the triode unused seems the most likely option at the moment.
If we would change the PI to a split-load PI we would free another triode and could remove one 12AX7. This would essentially leave a one-stage pentode-preamp amp (not much overdrive), which might be good for blues, but would be a different amp.

[tubenit]

It is not drawn neatly since it was done in a hurry.  However, for illustration purposes ......... does this represent the
latest thinking on this amp?  Feel free to edit it.

With respect, Tubenit

[Geezer]

I've not got the $ to buy an EF86, but I found a couple of NOS RCA 5879's in my stash, so I think I'm going to try that tube 1st in my build.
From the info I've found on the web & the data sheets, it seems to have slightly less gain than the EF86....possibly a good thing(?)

For the price of (1) EF86, I can buy dozens of the 5879's.........so the chances of finding a lower microphonic specimen will be better.

What'a ya think?

http://www.mif.pg.gda.pl/homepages/frank/short/001/5/5879.gif

http://www.mif.pg.gda.pl/homepages/frank/sheets/079/5/5879.pdf

http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/5879.PDF

[SoundmasterG]

5879's sound great and are cheap, so yeah its a great choice. Look at lots of Gibson circuits for how to use those, especially the '59 GA-40.

Greg

[Heinz]

EF85 and EF184 work well, too, but need slightly different part values. I have tested some and haven't had any problems with microphony.

I have finally found some time to continue my experiments with the prototype.
These are the results:

• the dual pot post-PI master volume is the way to go. It sound very natural at low volumes.
• I have reduced the gain of the EF86 by using a smaller cathode bypass cap and by splitting the plate resistor. The preamp is now controllable, even with the second triode stage re-inserted.
• I'm no longer sure about the cap switching. It does change tone, but not quite as you can hear in the demo videos. Either the cap values are off, or the whole circuit is wrong. I have drawn an alternative circuit that could do the job, but haven't tested it yet.
• The amp has that wonderful VOXy shimmer and sounds best when overdriven. It would be a nice amp even without tone stack and cap switching.

[tubenit]

In the interest to continue generating ideas to consider.  This would be like the D'Lite PAB but called "Bump" and the cathode caps on the second gain stage would be the "Bump Tone".   

I have used both of these ideas in amps and like the tone/gain changes & the simplicity of it.

With respect, Tubenit

[Geezer]

Just to keep this active & near the top....I'm starting the conversion of one of my "November" amps into the 5879 (EF86/pentode input) project circuit.

The topology is pretty much the same in the power section, so it shouldn't be too hard to convert.

I'm going to start with just the pentode > CF > Tonestack > PI configuration 1st, then try different things/ideas for the "Bump" feature (after I've got the "basic" circuit right).

[Geezer]

OK, got it wired with the topology I stated above (pentode > CF > Tonestack > PI) and it sounds good, but needs more volume & gain to the PI, so I'll add the 2nd triode driver before the PI & see what happens.

As for the tone, I am getting a "taste" of the pentode tone.....slightly compressed & very smooth & full, just needs more "push" into the poweramp.  ;D

I'm very excited & hopeful for good results!

Also, of the (2) 5879 tubes I have, (1) is slightly microphonic, but not so much as to be a problem (slight "ring" when tapped), but the other one is very quiet, indeed! The quiet one also seems to have a bit more gain to it....

[Heinz]

You want more gain? I'd want less... :o
Anyways, I love the sound of the amp!

What is your B+? And how did you wire up the pentode (bypass cap, plate resistor)? Could you please draw a quick-n-dirty schematic of the pentode stage?

[Geezer]

When I had the pentode connected as shown in your last drawing (split 100k's w/ the CF input from the junction of the 2x 100k's), the gain was WAY too low....in fact, very clean! & the overall volume was low, even with the vol pot all the way up.

I now have it wired as originally drawn/"traditional" (200k Rp w/ the CF input tapped from the plate of the pentode)....NOW we're talkin'! It still needs some tweaking of values, but it is very close to the tone in the video!

Current config = 200k Rp, 1Meg Rs w/0.1uf to ground, 2.2k Rk & 2.2uf bypass cap, 33k grid stopper.
Voltages = B+ node 255v, plate 105v, screen 65v

[Heinz]

I get the picture. You tried the 'normal' setting with the tone stack. I tried the 'bumped' circuit without the tone stack.

The tone stack reduces the signal roughly by a factor of 4. I reduced the signal by 2 with the split plate resistor and by 2 before the volume pot. That makes a total reduction by 2*2=4, so the circuits are similar gain-wise. Seems I tried to get clean tones from the 'bumped' circuit that it wasn't designed for. I'll change the pentode plate resistor back to the original configuration and attenuate the signal in the bump branch right before the volume pot.

I have tested my latest idea with the second triode stage and bypass cap switching, but it does NOT have the same effect as the level switch in the video. The coupling cap switching seems to be more likely.

It still needs some tweaking of values, but it is very close to the tone in the video!

Very interesting! My experiments indicate (to my ears) that the tone of the 'bumped' amp (tone stack bypassed) is very close to the video, as well. Obviously, we're not too far off..

Have you added the second gain stage, or does it seem unnecessary now?

[Geezer]

Have you added the second gain stage, or does it seem unnecessary now?

it seems unnecessary now, but we'll see once I get everything hooked up.......reworking the tonestack a bit now....

[CraigB]

HI, the March 08 GP magazine sports a picture of the SoHo chassis.  It's too small to glean everything, but I did notice that the MV is definitely a dual pot.  If there's anything else you'd like me to look for in the picture, I can try looking with a magnifier, although it sounds like you're getting close.  CraigB

[Heinz]

Thanks for the info! Could you scan it?

EDIT: found the article online http://www.guitarplayer.com/article/65-amps-soho/Feb-08/33519

[CraigB]

Yeah the article doesn't show the picture.  I will scan it and try to blow it up some.  If the file is too big, can I e-mail it to you?  CraigB

[Heinz]

There's a tiny pic in the article on the right, but it doesn't reveal much. You can't even tell resistors from caps...

I will scan it and try to blow it up some.  If the file is too big, can I e-mail it to you?

That would be very nice! You can send it to the email address from my forum profile (just click on the envelope below my name in this post). I'll upload it to my website and post a link so everybody can participate in the quest.

[tremolux]

Go here - http://www.tonecentral.net/gearpics/ down the list are several good Soho gut shots.

[Heinz]

Wonderful! Thank you very much!

[tremolux]

Been following this - sorry I didn't find it sooner!

[CraigB]

No need to scan the little postage stamp size picture I've got!  Those are great shots.  I wonder what the cap and resistor are there on the MV dual pot???  CraigB

[Heinz]

I'm currently trying to trace out the circuit. Unfortunately, the most complicated parts are in the most crowded areas. From what I've found so far I can say that that the MV is unlike the usual circuits.
Our ideas were close in some spots but completely wrong in others. I'll post what I have when I'm done.

[Heinz]

Here's as much as I could find out. Please check for errors.

Assuming I got it right...the circuit is certainly interesting. The MV switch adds gain by boosting a triode stage and the attenuator part of the MV strangles the signal before the PI. There's also some high frequency filtering going on, but unfortunately the component values are invisible.

The bump works by lifting the tone stack from ground, so we were pretty close with our guesses.

Why did they use a switch for the bump selector? Could it be related to power dissipation (especially with the bump enabled)?
And why did they use a triple switch for the MV instead of just lifting the MV circuit from ground?

[Geezer]

WOW....Thanks for that piece of work, Heinz!

It will take a while to digest all that.

In glancing at some of the missing (?) values, some of them, at least, can be guessed at by the surrounding component values...others will just be a "try & see" approach.

[Heinz]

Minor Update: I corrected some drawing errors.

[Geezer]

Well, I'll be re-wiring my test bed for the pentode>triode>CF>TS>PI configuration.

The pentode>CF>TS>PI sounds "good" in it's own right, but not like the SoHo.

BTW, doesn't that newly discovered configuration look a bit like a 5F6-A/JTM45 circuit w/ a pentode?

[Heinz]

I changed my prototype to reflect the new knowledge - and it sounds just great! 8)

The bump circuit works as expected (boost factor, level and tone control) and the normal tone controls provide the usual range. I'm not sure if I got the master volume circuit right. I've been meditating over the pictures for some time, but I can't see for sure whats behind that resistor on top of the pot. I'll go about this later.

I have compiled a new schematic with the component values I expect to work. The .sch file has a second sheet with my prototype (call it the 'SoHeinz'  ;)) which includes some simplifications to the bump circuit. A pair of 6BM8s saves one ECC83 and makes a nice 4-tube amp with good sound and decent volume (still enough to make my wife complain).

[Geezer]

I rewired mine to match your last posted schem (just the basic circuit....no bump or MV).

Sounds pretty good with the volume low, but it either goes into cutout OR starts oscillating @ ~ 1/4 way up.

If I severely drain some of the signal after the volume pot (39k resistor to ground) then it calms down & sounds really good.

I'll be trying more tweaks today....may be lead dress, since this is patched together in an older build(?)

[Geezer]

Wanted to say that there really is a difference in tone of the pentode compared to a triode (even a paralleled dual triode, which I heard some guys compare pentode inputs to).

It's kind'a "jang-ly" sounding & responds really well to pick attack....cleans up with a light pick, then distorts when dug into! Also responds well to the guitar volume, and it's a "smooth" type of compressed overdrive, but still retains definition....I like it alot!

[Geezer]

Heinz, I am supposing that the screen resistor on V1/EF86 is mis-labeled? You have it as 470R, but that doesn't work well with my 5879....I'm thinking it should be 470K(?)
Even that value doesn't sound good on mine, so I have it back to the 1M.

[LooseChange]

Geez, Mine has a 2.2M
The Pentode preamp tube is fat and jangled. I have an amp with a parallel triode and a EF86.  I switch between them.  The Pentode is the lead channel.

[Heinz]

Oops! Of course you're right, it should be 470k. The 470k/1M resistors make a voltage divider that keeps the screen at about 2/3 of B+. The 100n cap provides some stabilization to the screen voltage. This circuit has a gain of about 100-120 and a voltage swing of nearly 180Vpp (B+ 250V) before it distorts visibly on the scope. You can see the compression set in at about 100Vpp.

I haven't seen this arrangement before, but it works about the same as the 1M/100nF combination. In that case the internal screen resistance is the second half of the voltage divider. A 2.2M resistor will set a lower screen voltage. If the plate and cathode resistor remain unchanged, this will result in a reduced output signal voltage swing which means it will need less input signal before it starts to distort. This may be what you want...or not. Maybe a guitar pickup signal can't even drive it to this point.

The AC15 uses a 220k plate resistor, 2.2k cathode resistor and 1M/100nF for the screen screen.

[Heinz]

I'm still trying to figure the master volume circuit out. Here's an enlarged piece of the gut shot with some remarks.

Unfortunately, there's no way to look behind that brown resistor that sits on top of the pot. There are two wires coming from behind that resistor, but I can't see what type of component it could be. The black blob is a cap that is probably connected to the left lug of the pot but where exactly does the right leg go to?

Could my schematic (see above) be correct?

[Geezer]

Heinz, the Rk on V2A......is 47k correct? That's really cold biased & takes all the life out of the tone on mine.

On the master, your guess looks pretty good, but there does seem to be some other component under that brown resistor....almost looks rectangular, like a wima type cap(?), but that doesn't really make sense, since they used Sozo caps everywhere else..... :-\

I'm REALLY getting frustrated with my attempts to get this "basic" circuit right (no bump or MV on mine yet).
It sounded better to me without the extra triode (although we now know that the Soho uses it).
Mine has way too much gain (ocsillates like crazy if turned up), is too "chimey" (too much highs), is too boomy without the bass turned all the way down....I need to step back a while & try to get my perspective straight again....  :(

[tubenit]

DaGeezer,

I am wondering if some of the oscillation is from the complexity of the build & layout?  I would be tempted to try something like this which has the mstr vol prior to the LTPI. I would guess (maybe incorrectly) that it would be less prone to oscillation?

With respect, Tubenit

[Geezer]

Thanks T, I am thinking I will like that arrangement & will try it.

I found this article ( http://www.freewebs.com/valvewizard/pentode.html ) that describes that the "standard" values given for pentode operation (Vox AC15 & such) is for absolute max gain, & gives some suggestions & guidelines for values more suited for guitar use......

[Heinz]

The 47k cathode cap for V2A is just a guess. The component is not visible in the pictures, just its connections. I get too much gain if I use a substantially lower value. With the 47k the range goes from clean to very distorted, just as it should be. Choose an appropriate value if this value doesn't work for you.

One important aspect my prototype seems to be the B+ voltage. It needs 240V or more for the preamp, otherwise it does not clean up, even when the guitar is turned down.
Strangely enough, mine is very bright and sparkling. Could the different pentode type you are using make such a difference?

[tubenit]

One thing to think about is that it's possible for an individual to get used to a mental picture of the way things should "BE"!   And I continue to realize within myself how much of how I approach music is a mental thing.

My Carolina Blues Rocket will oscillate if I crank everything to 10. That really bothered me at first but I "got over it".  It's a high gain amp.  I will NEVER play everything cranked to 10.  I discovered the most that I like (and will tolerate) is vol at 10, gain at 7 and mstr vol at 5. That's it! It's unbearable to me beyond that.  AND .......... I have to crank it beyond what I would ever use it at to get any oscillation.  And I play it waaaaaayyyy less cranked than that with the band I play in.

So I adjusted my thinking ........... my amp does everything I need that amp to do & I enjoy playing it.
I realize if I were trying to sell one and stated "you can't crank everything to 10 or it will oscillate" then I might not have any buyers.  But the amp is for me.

Just my two cents worth!    With respect, Tubenit

[Heinz]

I have played with my prototype for some time now (my ears are ringing  :o) and tried both my guitars (a yamaha pacifica and an ibanez LP-type with active pickups). It seems that the amp likes humbuckers better than single coils. You'll also notice that Peter Stroud always demonstrates the amp with a LP.

That (nice) bright jangly tone appears if the signal level exceeds a certain value. If either the guitar or the volume is turned down too much it sounds somewhat dull. Maybe the reason why humbuckers sound better is simply the higher signal level.

If you want to reduce the bass you can use a smaller cap value for the 470k/10nF pair right after the CF. I've been wondering about this cap value (it's more of a 'broad' cap than a 'bright' cap) but it sounds fine in my amp. Reducing this cap may also take away some of the extra gain.

I have also installed two 47pF caps in parallel to the 220k grid resistors of the power tubes. You might want to try this to reduce the oscillation.

[gregarious]

da Geezer quote:

"On the master, your guess looks pretty good, but there does seem to be some other component under that brown resistor....almost looks rectangular, like a wima type cap(?), but that doesn't really make sense, since they used Sozo caps everywhere else....."

Like to say upfront that I don't know diddly (but given have a chance would love to meet Bo), so I searched on the physical appearance and found RFI suppression caps which have a similar footprint to the appearance of the component back of the pot, some are available with a metal sheath over the plastic case.

http://www.evox-rifa.com/technote_pdf/rfi_fact.pdf

http://www.lcr-inc.com/pdf/MEY-Y2.pdf

Whether it makes sense to place one in this section of the circuit is beyond me.

Just a stab in the dark....