Speaker question ?

[SpareRibs]

Hello,
       Like I  said before I know I ask a lot of dumb questions, however I had an idea and was wondering if it is possible.
       I would like to wire two 8ohm speakers to an amp. Is there any way to put a split second delay on one
of the speakers, so that it is transmitting the same signal only a split second later? I don't know if it is at all
possible, or how much trouble it would be even if it could be done.

[J M Fahey]

Put one close to the amp, the other 30/90 ft away.
There's your delay.

[ChewyNasalPrize]

I imagine it could be done with some sort of circuitry between one speaker and the other to delay the signal but I wouldn't know what that would be. Might be easy, might not. If you do it, I'd rig it so you can bypass it with a switch. I doubt you want permanent delay/echo effect on your amp. But whatever turns you on is what matters. :dbtu:

[Roly]

The only dumb question is the one you fail to ask.

Apart from a very long speaker lead to a box in the parking lot,    the normal way to do this would be some sort of "low level" delay device - a delay pedal,  MIDIverb, etc., and a second amp to drive the delayed speaker.  There is no simple, or even complex, way to insert a significant delay at what we call "high level", i.e. between amp output and speaker.

[SpareRibs]

Hello,
      The delay was just going to be another way to try to get the amp to sound a little more tube. It is not necessary.
      My main objective was to get the amp to sound as close to a Fender Champ as possible, without all of the constant tube changes, biasing, and all of the other upkeep required maintaining a tube amp. On some of the other sites it seems like people get obsessed with just that. I just want reasonable sound you can just turn on and turn off, without rebuilding the amp on each occasion.
      I do not use pedals and such. I have a Squier Telecaster, and an Epiphone Les Paul JR. I like to just run them straight into the amp. Those guitars are very basic and crude, and so was the Fender Champ.
      I have 18 other guitars, and a Crate GX-40C+ with chorus and reverb. I just really like the raw gritty sound of low tech instruments and electronics from the 50's and 60's.
      The diode mod and the 12" speaker really got it in the ballpark, I am well pleased with both those changes.

                                                                   
       

[SpareRibs]

Put one close to the amp, the other 30/90 ft away.
There's your delay.

Hello,
      Sounds good, by the way could I mail one to you with a remote transmitter attached to get a really good delay ?

[Enzo]

Please do experiment as much as you can to learn, but always remember, this simply is not a Champ.   Imagine a restaurant that sells lobster.  And a chef trying thing after thing to make his lobster taste more like steak.

[SpareRibs]

Hello,
      OK, Enzo I understand. As good as it sounds now will just have to be good enough considering what it is.
Thank you.

[SpareRibs]

Hello,
      Is this something to work with ? Someone on another forum said it was capable of being converted to a 5e3. At least it is tubes and some of the same components. Since it is tubes should I post about it on another forum. 

[Roly]

Generally speaking the problem with converting a s.s. amp to valves isn't so much electronic as physical.  If you look at a few amps of each type you start to notice that s.s amps tend to be built on one side of a U-shaped chassis while valve amps tend to be built on both sides of an inverted U-shaped chassis.

It's not that the problems of the physical layout can't be overcome, but after a bit of head scratching you generally come to the conclusion that it would be easier in the long run to make a whole fresh start with a case and chassis arrangement intended for valves and build your valve amp on that.

From what I can make out of your Newcomb circuit it's for something like a factory PA and I'd guess from the valve lineup and drafting dates from the late 50's to early 60's.  Even modified (and it would require a fair bit of modification) I don't think you would find this a very satisfactory guitar amp.

[SpareRibs]

Hello Roly,
    I am not talking about converting the SS to tubes. I am wondering if the Newcomb would be a better choice for modification. I have ran a guitar through it and it is loud. It seems to have to much gain. I can understand
why as it was probably more for talking as opposed to amplification of music. I am going to make a cabinet as
I have one 12" 16ohm speaker. I will just see how it works. I am not disregarding your advice, I know it will be
a problem. I appreciate all of your input and all of the advice you have given me in the last few weeks. Thank
you very much. 

[Roly]

Ah, sorry, I misunderstood; you already have one of these and want to modify it.

Well as it happens there are a line of AWA/STC factory PA's that were made here ranging from a few watts up to 120W which are very similar and many of which have been pressed into guitar service.  Tim Robbins is a great bloke who has done many like this and you can contact him here http://dalmura.com.au/projects/amplifier.php.

The circuit is very small and blurry (or maybe it's me that's blurry  ) but an important point is what is available on the output transformer secondary.  Many of these were made to drive "voltage line" rather than low impedance speakers, but it looks like this one might have suitable low impedance tappings.  If not there are often suitable sections of the winding that can be found between tappings.

While it may seem to have too much gain now, particularly if you go in via the mic input, you will want to fit some sort of tonestack, the Big Muff being a single-knob improvement on the simple top-cut fitted,  and this will have about 20dB loss so the extra gain will tend to be compensated that way.

The input transformer for the microphone won't be required, but put it to one side because these can be useful to provide an isolated line out for recording etc.  It will have a mu-metal shell that should be treated like glass.

The Phono input should have about the right sensitivity for clean guitar, but you can press the mic preamp into service if you want a "crunch"/gain stage, but the component values will need to be changed to something a bit more conventional, say 1Meg on the grid, 100k on the anode, and about 1k2 on the cathode, bypassed to taste with around 25uF.

The input coupling cap is a pretty small value and may need to be fattened up to say 0.1uF, and taken direct to the input jack tip.  Ditto with the other coupling caps, but then I expect that these will be the dreaded waxed paper types which will now be leaky and require replacement anyway.  The interstage caps need to be HT rated.

Three core mains lead with a secure ground to the chassis.

You can experiment with opening the 180k negative feedback resistor path for possibly a bit more "tone" (i.e. distortion).

The 100k pot in the middle is to balance the drive to each of the output valves (it's a slightly unconventional Phase Inverter, but no matter if it's working).

Naturally you will have to evaluate the health of the electro caps on the HT line which must be about 50 years old by now, the rectifier (which may be replaced by silicon diodes), and the 6V6's for output.  Until you are certain of the electro caps use a limiting lamp.

Go over all the resistors too, and replace any that are more than 10% off value with one end disconnected.

But yeah, overall it's a very plausible project.  Should give a very respectable 14-odd watts with healthy 6V6's.

HTH, and keep us in touch.

[SpareRibs]

Hello,
      Roly, I have replaced all of the caps just because of the age of the unit. I turned it on and it was so quiet I thought I had left something disconnected. I brought this one to our local amp repair person for him to check out. He told me he can only go through circuits and find problems, and that he is not an engineer. He does a wonderful job of troubleshooting and repairing problems, however he cannot discuss theory and tell you what will happen if you do or don't do this or that. Being that he is the only person knowledgeable in repairing tube type stuff in this area it is quiet frustrating at times. The only other option is going to the big city to have one of the people there tell you they don't know either.
      I am going to pull it apart again and look at it. I will likely make some of the changes you suggested. At least this one works I just have to tone it down a bit. Sorry about the schematic. I pulled it off the web
because while there is one  inside the amp I would have to take a picture of it, download it from the camera
upload it to the computer and then post it in this forum. That could take most of the day and may not be as good as the one you are looking at now. I was actually surprised the schematic popped right up on the web.
      Anyway thank you for all of your help so far. I am sure I will have more questions soon.

[Roly]

Okay, assuming that you have something like an 8 ohm tapping on the output transformer and you have a good speaker connected to it, and you can power it up via a limiting lamp without smoke...

Are the valves lighting up?

Have you got 2-300V of HT coming out of the rectifier?  Is it getting to the OPT centre tap?  Is it getting to the 6V6 anodes? (you will need the data sheet to identify the valve pins)

How much voltage is across the common cathode resistor after the valves have warmed up? (this tells us how much current is, or isn't, flowing in the output pair).

When you probe the 6V6 grid pins is there any sign of life from the speaker, faint scratches or pops?

If so, what are the voltages on the two Phase Inverter valves, anodes and cathodes?  Do you get (louder) scratchies when you probe either PI grid?


I won't take it further at this point, but the idea is to first make sure the voltage supplies are sufficient, then starting from the output working backwards along the signal chain looking for signs of life from the speaker at each stage, sorta like finding a blockage in a pipe.

{By the time you're done with this he'll be bringing problems to you}

HTH

[SpareRibs]

Hello,
     Roly, I don't think I have to go through all of that. I worked on it in the last few days. It is coming along well. I have a Fender Tweed Deluxe tone stack in it. I don,t know if that is the best one but I found a schematic for it, it may need changed or modified at some point. Would the volume need a 470K resistor in front of it to load the guitar pickup ? Some guitar amps have a .68-100K  in that position.
     I changed the grid resistor from 18K to 1.4K that was the lowest value I had. I also added another resistor in parallel to the 560K plate resistor 510K, it should be roughly half now.
     I fired it up today and it sounds much better. Some of the gain is gone, however it is still leans a bit  towards the bass side. I believe I can work on the caps to iron that out. I also think a better speaker will be in order when I get it operational.
     So I am still just swimming around in the dark, but its getting better. Thank you once again.