Need PA amp conversion help

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Hello All! I'm new to the forum and also mostly new to amp building. I'm looking for some input/help on the following from all members experienced in converting old PA tube amps into guitar amps. Any help given would be much appreciated!

I have a Webster TP45, 30 watt tube PA that I've restored to original working condition + adding a grounded power supply, a guitar jack input in channel 1 and an output jack and the speaker send for ease of running into guitar cabs. She fires up on all tubes, has all channels working, and sends out a LOUD, clean, and clear signal to the speakers. I've attached a schematic with the signal path of the mic 1 input highlighted. It's really a very good and clean PA design. Now to mess it up and make it a good guitar amp design lol.

Running the guitar into the amp as is works okay, but it leaves a bit to be desired as a guitar amp. It's very full frequency, but still kind of on the sterile sounding side. Also you couldn't make this amp overdrive with a guitar signal no matter how hot the pickups are. Of course as a PA it was designed to defeat distortion and remain clean to top volume. On the plus side, this sucker is the loudest 30 watts I've ever heard in my life. I'm a gigging musician and not squeamish about volume, but going into a 1 x 12 open back cab, I can't stand much past 3/10 on the volume before it starts to get unbearably loud. I'm pretty sure the very low resistance on the negative feedback line helps contribute to the super loud volume.

I've also attached a scan of the modifications I'm proposing to make this better for guitar. I could sure use some help/guidance on these changes to see if I'm on the right track or if anyone sees something off in my changes or knows a better way to do this. I'm hoping to achieve the following:

1. I'd like to keep the basic design in place, but change the values of the caps and resistors for mic 1 channel to be better suited for guitar. I'd also like to do some similar light modification to the output section to make it more prone to breakup when driven.

2. All three input channels share a 750ohm/200uf cathode. I'd like to separate channel one from this so I can voice it better for guitar, but leave the other 2 inputs as is for now. Can this be done by placing the 2.2k resistor and 22uf cap where I've added them?

3. Adding 68k grid stopper and changing the other resistor to 1m makes this first gain stage look a bit closer the old 1960 Vox AC30 ef86 normal channel schematic. I think this is a good place to start no?

4. Adding 1k grid stoppers and 470k screen resisters at the 6L6 power tubes to get the power section to break up a little sooner.

5. Increase resistance on the negative feedback line to help the amp overdrive sooner, and also maybe slightly decrease the overall volume.

6. What did I miss, anyone see some other obvious changes they would make that I'm not addressing here? I think the rest of the circuit is good for guitar: 2 EF86 gain stages, a James TS sandwiched between 2 triode gain stages, into a cathodyne phase inverter, into the 6L6GC power tubes... it's a bid of a long chain for a guitar amp but we should be all good here right?

Thanks again in advance for any responses!
 

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Suggest you measure what feedback level is being applied - what measurement facilities do you have?

Also the PI grid is likely going to need a nice large stopper, as it will get the most overdrive. There also may be no need, and probably a disadvantage to having a low bass response through the 0.1uF coupling caps to the output stage.

If you aren't going to use the multiple inputs, then I suggest disconnecting the 1Meg mixers, and dropping the direct path down from 1Meg to reduce noise.

It may be worthwhile testing what your output transformer provides for taps.

Safety is also a concern for any vintage PA : https://www.dalmura.com.au/static/Renovating%20PA%20amps.pdf
Ciao, Tim
 
Tim, thanks for the input! I only have a multimeter at the moment. No other instruments for measurement.

You’re proposing a grid stopper going into the PI to assist it in ovwrdriving quicker?

On the 0.1 coupling caps, the bass can get pretty heavy on this amp already. I worry romoving those caps might make the bass a little too boomy.

I am thinking of leaving the other 2 inputs and voicing them later for other instruments/inputs. Also aren’t those 1m resisters doing a bit of attenuation to the signal to going into the mixer? Would I need to worry about too much gain after removing them?
For right now though I’m going to leave both other mic channels intact and see how it sounds. If I decide to use only 1 input channel I may revisit this. Also I’m not too worried about noise as this amp is a quiet as a ghost, it could almost stand to have some more noise to give it a bit of character lol.

Thank you for the reminder on the safety. I’ve done my due diligence to make my work area and practice as safe as possible, but a friendly reminder of the inherent dangers of tinkering with old amps is always a good thing!
 
Doing any tube amp work typically necessitates measurement capability beyond a multimeter, especially if you are modifying circuitry, as is often the case with a PA amp conversion.

Can you generate a signal from a PC soundcard (line out socket), or an iPod or headphone USB/memory mp3 player, or CD player or tape deck - and use it to provide a sinewave signal input to the amp?

Your multimeter may have specifications that show its ACV frequency range. Use a sine frequency that is within its frequency range.

Google how to check negative feedback level and find a dB calculator to converter measured voltages to dB gain - use an input level that generates a low output speaker level eg. 1Vac for 8 ohm resistor load (use a resistor rather than a speaker). Disconnecting the 13k feedback at one end will cause output Vac to rise - then calculate dB rise - that is then the dB level of feedback being applied in your amp.

An amp will start to clip the top of a sinewave at some input signal level. Hi-fi amps don't want that clipping to occur anywhere within the amp. Guitar amps often have gain and volume pots that allow a few stages to be the one that clips first, or a few stages at about the same input level. Clipping can cause a rapid rise in distortion, so typically those stages that can be allowed to over-drive by grid conduction (approaching 0V from grid to cathode) use a grid stopper to soften the grid conduction characteristic. That is a main reason why your schematic shows a grid stopper for input stage and output stage valves. Google grid stoppers for cathodyne PI stages.

A coupling cap and the following stage's grid leak resistor form a high-pass filter with a corner frequency that is often below the audible range - your 0.1uF / 150k have an 11Hz corner, which is fine for hi-fi or wide range amps. However, guitar amps often aim to overdrive the output stage, which can cause a somewhat distasteful form of blocking distortion, which is typically tamed by raising the CR corner frequency up towards or above the guitar lowest note frequency - so circa 80-100Hz - so you could drop yout 0.1uF coupling caps to circa 0.01uF.

Your schematic incorrectly labels the 6L6 grid stopper as a screen stopper - the 470 ohm 1W is the 'screen stopper'.

If you can't loan a scope (or buy a small modern cheapy), then many nowadays are using a PC soundcard with some software to achieve an oscilloscope function, as well as imho a much more useful spectrum analyser function. For simple testing, there is no need for a high performance probe and soundcard and PC, and there are a few on-line references to show that, although they can quickly get quite elaborate which is not really needed for starters. For example, if you are only probing the speaker output, then there is no need to setup a 1 Megohm input probe, and a simple resistor divider can attenuate down to a level that won't damage a soundcard input. And using a battery powered cd player or USB headphone player or iPod will avoid one grounding and ground loop concern.
 
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Tim, I appreciate the input on using an audio device to send signal and a 8 ohm resistor to send the speaker load to for measuring the negative feedback level. I will definitely look into that this weekend.

So you were suggesting the grid stopper going into the PI as well to be sure it overdrives like the preamp and power amp sections and doesn’t turn into high level blocking distortion? What value would you use there?

I see, you weren’t suggesting removing the coupling cap in front of the the power tubes, but rather changing the value to cut more low end out of the signal? Okay, I can get along with that.

Yes I’ve seen some tutorials online and places to download software similar to what you have mentioned. Definitely need to look into this. Thanks again!
 
Running the guitar into the amp as is works okay, but it leaves a bit to be desired as a guitar amp. It's very full frequency, but still kind of on the sterile sounding side. Also you couldn't make this amp overdrive with a guitar signal no matter how hot the pickups are. Of course as a PA it was designed to defeat distortion and remain clean to top volume.

You got it in one. No decent "guitar" amp is designed that way. Firstly, the frequency response has a big mid scoop and rolls off rapidly outside 100Hz and 4kHz (you speaker will help here too).

The other bit is that almost all guitar amps have one or more pre-amp stages that are "badly" designed, so that you get asymetrical clipping long before you get output stage clipping.

How you arrange that happening gets you anywhere from Supro to Soldano

I'm a bit busy (and a lot lazy) so I'll refer you to three sources
a) Gnobuddy's recent posts in this forum. He outlines the above in great detail.
b) Rob Robinette's amps pages, with many worked examples
c) Merlin aka Valve Wizard.
 
thoglette, Thank you for the response and the references. I'd already discovered Rob Robinette's site and also the Valve Wizard site. Those references have been very helpful in making my redesign thus far, but Gnobuddy's post could be helpful too so thanks for that.

For everyone else who's also contributed or following along, my latest update is that I got my start working on making the amp changes Sunday. I was able to finish soldering in resistors and two guitar jacks at the input stage. I also removed the phono line and located the point I need to insert the old phono volume pot to make it a master volume. Then I located the insertion point for the 1.5k grid stoppers and 470 ohm screen resistors in the power section. That one will take some tricky rewiring and soldering to fit everything in there correctly. There are a lot of tight spots in the exhisting point to point soldering, so it's a bit tricky and slower going than I'd like. I'll be working on it again next weekend and hope to finish it up. With a little luck I'll be able to post a vid of the results in the next week or 2 for anyone interested in how it turns out. Thanks again all!
 
but Gnobuddy's post could be helpful too...
I'm not sure I've posted anything useful to you recently, but I just took a look at your schematic, and there are a couple of issues I spotted.

First, the resistors you've marked "470k", connected to the screen grids (pin 4) of the 6L6 GC should 470 ohms, not 470k (470k is a thousand times bigger than 470 ohms, so this is a show-stopper unless fixed.) Perhaps you meant to use 470 ohm, and 470k was a typo, but I wanted to make sure you were aware of the error.

Second the 1k resistors you marked as "screenstoppers" are actually connected to the control grid, and are "grid stoppers". These do not need the high power rating you specified (5 W), ordinary half-watt resistors will do. And you can safely increased the value considerably beyond 1k - I would suggest something like 15k as a starting point.

(Bigger grid-stoppers on the output valves help to reduce blocking distortion when you do succeed in overdriving the output stage, and also helps out with high-frequency stability of the output stage.)

I know working on a complex amp built point-to-point has got to be challenging. But on the plus side, you have a treasure-trove of valves to play with! With four triodes, it looks as though you could build something Fendery without ever using any of the 4 EF86's, if you wanted to.

Good luck with the EF86 input stage. Those have a bad reputation for microphony problems in guitar amps, but I guess you'll soon find out if that's a problem in your build, or not.

If you do end up with microphony problems, you might consider changing the input EF86 stage to lower its voltage gain. But we can cross that bridge if we come to it.


-Gnobuddy
 
Gnobuddy,
Hey guess your ears where burning haha! Thanks for dropping in on my thread. I had posted a new updated schematic with the correct470 ohm values on the screen resisters and a few other changes. I’d also figured out my typo in labeling the scrgrid stoppers at the going into the power tubes, thank you for the input though!

Yes it is a challenge to work on but also fun. And right you are on the abundance of tubes! My backup plan if I can’t get this to sound good is to do what you mentioned and gut the preamp section to reuse the triodes and 6L6 power section for something akin to a bassman/bandmaster kinda setup. But I want to see these adjustments to the original circuit through first in case I can make something more unique sounding but still good. Lots of fender wannabe circuits out there, but no circuits with pentode preamps and 6L6 outputs that I’m aware of. Some pentode preamp to EL34’s sure, but not 6L6’s . Of course I may find out there is a reason for this lol.

On the infamous EF86 tendency for micophony, I am aware and have a secret weapon to combat this with. I found a local amp builder who makes conversion basses that will plug into an EF86 socket And accept a 6AU6 pentode which is less prone toward going micophonic and also much more abundant and cheap to buy. Worked great when I tested the amp after restoring it to spec (before I started the conversion to guitar amp). Great hack and the tube properties are close enough that the amp doesn’t know the difference. The builder I bought the conversion bases from has used them successfully in Vox AC30 models with the old style EF86 normal channel that is notorious for going microphonic.
 
...I had posted a new updated schematic...
Sorry I completely missed that! :eek:

It sounds as though you've already thought this through quite a bit. I'm rooting for you to succeed, and end up with a unique amp you like.

I do have a few thoughts to offer:
1) The triode stage before the cathodyne is missing a grid leak resistor. Yes, the master volume pot provides that function, but potentiometers eventually wear out, and the contact between wiper and track becomes intermittent. And when that happens, that triode will have nothing to control its grid potential. So I suggest adding the usual 1M grid leak resistor to this triode.

2) Exactly the same issue also applies to the triode connected to the bass (tone control) pot wiper.

3) The bias voltage on the 6L6 control grids - shouldn't that be adjustable, so you can set the bias to match the tubes? It almost looks as though some components are missing from that bias network. Have you measured the grid bias voltage (between pin 5 and pin 8 of the 6L6s)?

4) It's not a bad idea to insert a 1 ohm resistor between each 6L6 cathode (pin 8) and ground. You can use your DMM to measure the DC voltage across each of those resistors, which will tell you what quiescent current each 6L6 is biased to.


-Gnobuddy
 
Gnobuddy,
I have done quite a bit of research, I never do anything halfway lol. I've also had a LOT of help from kind people such as yourself here and on another forum. I'm learning on the go here so I will take all the help I can get!

Good advice on the triod grid leak resisters, I had read about the master volume being good enough resistance going into the PI triode, but the pot wearing out over time makes sense, especially on a knob that will get a fair amount of use. The 1M resistor should go after the volume pot in the signal chain right?

I hadn't even thought about a grid leak resister for the first triode stage with the tone stack, good catch! I have plenty of 1M resisters so I will be sure to add one here as well.

Unfortunately there's nothing missing on the schematic, it is a fixed bias but with no pot or anything else for adjustment. From what I have read this isn't uncommon in these old PA amp designs. I guess they just selected tubes on the colder side and rolled with it lol. On the upside, the guy I bought the amp from had already taken it to a tech in an effort to get it working. The tech measured the bias and by process of elimination selected two brand new matched tubes that are as close as he could get to the bias set in the circuit without going too hot. Long story short, the owner gave up on the amp shortly after setting his eyes on a different easier conversion and I ended up getting this thing for a good price. I do intend to insert a pot for bias adjustment in the future, but that is much further down on my list of things to do. I think I should be good for at least the life of the current tubes. *fingers crossed*

Thanks Again!
 
...the pot wearing out over time makes sense...
Defensive designing - it's a bit like defensive driving. :)

It may not matter a whole lot in this particular case, but when it comes to a pot that sets the bias current for the output valves, failure of that $2 trimpot can cause destroyed output tubes, output transformer, maybe power supply transformer and tube rectifier as well.

But if we plan ahead, we can design the bias network to fail safe - if the trimpot fails, bias voltage should go to maximum (negative) grid voltage, so the output valves turn off, rather than turn bias full on, and cook to death.
The 1M resistor should go after the volume pot in the signal chain right?
Right!
...a fixed bias but with no pot or anything else for adjustment.
<snip>
..I do intend to insert a pot for bias adjustment in the future...
Interesting! Maybe power tubes were more consistent in the old days, so they could get away without adjustable bias?

I very briefly owned a Blues Junior that had the same sort of fixed bias arrangement. I bought my Blues Jr. second-hand, and when I opened it up to check it out, it was clear that the EL84 output tubes had already overheated badly. The white lettering on the glass had been burned to brown. One glass envelope literally pulled apart in my fingers as I was gently pulling the tube out of its socket. :eek:

I learned two things that day: fixed bias with no adjustment is a bad idea, and always wear heavy gloves when inserting or removing tubes from their sockets!

One of the first things I did with that amp was to design an adjustable bias network for it (a trimpot and a few fixed resistors), and add those components to the PCB.

When the time comes to modify the bias arrangement on your amp, maybe I can help. (I think you'll need to change the bias rectifier to a voltage doubler so you get more available bias voltage than you have now, and then add a bias trim-pot and fail-safe resistor.)


-Gnobuddy
 
Interesting! Maybe power tubes were more consistent in the old days, so they could get away without adjustable bias?

That's probably true, but also I'd theorize that it also had to do with the fact that tubes were much cheaper and in abundance when the amp was designed, so finding 2 tubes that were just on the colder side was not a difficult thing to do. Also I'd imagine that, in most applications for a PA, having it run on the colder side does not negatively effect the output in such a noticeable way as it might for a guitar amp.

When the time comes to modify the bias arrangement on your amp, maybe I can help. (I think you'll need to change the bias rectifier to a voltage doubler so you get more available bias voltage than you have now, and then add a bias trim-pot and fail-safe resistor.)

Sounds like a great idea, I may indeed take you up on that. Thank you! :)
 

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> our side of the Great Pond people in the 1950ies had to work much longer for the same tube type than we do today.

The prices I paid 1960s-1970s were more, relative to gasoline and meat, than I pay today.

I think "bias adjust" is just something to keep today's hobbyists busy.
 
I think "bias adjust" is just something to keep today's hobbyists busy.
You mean, because every factory always gets it perfectly right, every output valve is identical, and the AC mains voltage never ever changes as the decades go by? :rolleyes:

We already know the mains voltage has changed considerably since ElusiveMoose's amp was originally designed. It will be biased colder now than it was originally. Maybe the increased anode voltage will perfectly compensate; maybe EMoose will win the lottery, too. Why would anyone not put in a trimpot, and guarantee the proper outcome?

In stock form, the EL84s in the Blues Jr. I had got so hot that the glass envelope of one of them fell apart when I pulled gently on the valve. The crack occurred right where the metal valve retainer meets the glass bulb. Presumably the excessive thermal gradient where the too-hot bulb touched the metal retainer had already started a hairline stress fracture there. The dark brown lettering that had started out white confirmed that the valves had been running far too hot.

If you do a little Internet searching, there are reports of EL84s biased to over 100% of datasheet maximum anode dissipation in stock Blues Juniors. At least 30% too hot, straight from the factory. Fender's engineers didn't get it right. Given production tolerances on valves, they probably couldn't get it right for all of them.

Factories get some things right, and some things wrong. Likewise with hobbyists. Being completely dismissive of one group or the other is just silly.


-Gnobuddy
 
Finally, results!!

Alright, managed to get some real work done this past weekend and finally have some results to post! Attached is an updated schematic showing all the changes to the circuit that I have completed. Also here are links to 2 short videos of me testing the amp in its current state with some single coils and humbuckers. I'm pretty happy with the results so far. :hphones:

YouTube

YouTube


The next order of business I believe is to make some adjustments to the Tone Stack. I'm happy keeping the basics of the current TS circuit in place, it is essentially a passive James/Baxandall TS, but the values of the capacitors around the tone pots are not ideal for guitar signal. I managed to get some decent tone out of the current set up, but required some very extreme settings between the bass and treble pots to do so. The bass is overwhelming everything unless I cut it out almost entirely.

Can anyone suggest some new capacitor values to start with that would allow the tone controls to make less high-fi frequency adjustments and more focused adjustment to the 60hz - 10khz range? FYI, the bass and trebble pots are both 2M, this seems very high for tone control, should I change those out as well?
 

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