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Hello inmates,
need some major help and I also might need some fundamental replies as well.. I posted earlier this month about a pair of Wurlitzer 6420 6L6 organ amps I picked up for a reasonable price.. I have one of them rebuilt and converted to audio use. It's actually not too bad and I didn't exactly put the most pricey parts or tubes into it. These are a cathode self biased amp being driven by a single 6SN7 input. I think these are rated for 18 watts, but when I bench tested them I managed to get a respectable 28 watts out of it..
Pretty pleased with the modest results so far, however if there is one issue, it's the amp needs some better gain. This was an issue suspected to be the case when I first posted.. Well now that I have one of them running properly, Mikey Samra suggests I build a "gain stage" and then pipe into the existing 6SN7. He suggested using either another 6SN7- which I like because I have another one of those tubes I can experiment with, or a 6SL7.
I am not exactly sure how to do this, nor am I exactly sure of what I am doing by creating a gain stage, but it looks like I can steal some power off the b+ which is around 450 and head into another tube, which I can mount since I wont be using the old speaker sockets, I can install a tube socket in where these are..
Mikey says to look for a similar set up and copy the schematic, but I have not had any luck finding something similar or knowing if it's in fact a gain stage I am seeing.. I was wondering if anyone out there would either know how to do this, or be able to point me to a schematic I could copy directly or with minor changes..
I am posting the schematic that Fender Lover cleaned up for me and two images of the amp.. I need "novice" bone-headed simple direction..:)
thanks for any help
Jeff
Follow Ups:
update...
so I borrowed a guitar and brought it home and plugged it into the amp...
The good news is that it sounds wonderful, clean, nicely rounded...in fact I think it sounds better as a guitar amp than an audio amp...
The bad news is I would say with the pick-ups turned up to full volume, I would estimate that the amp is maybe a 1/3 of it's full potential..
So not sure if converting to a 6SL7 would get me all the way there, or if going back to the idea of making a gain stage is the right avenue..
that's where I am at so far...
Jeff
-
no pre-amp, just plugged in direct to the amp..
J
You need a preamp to use that with a guitar!
I don't think you need any more gain unless you are turning this chassis into a guitar amplifier!
The plate to plate impedance seemed in the right area. Funny though, my chassis are marked for 6V6s, even though the parts inside and outside look exactly the same as yours!
that's odd, I wonder if year of manufacture plays a role.. It's hard for me to determine the year these were made.. but as you can see in the image, I am even testing them with 7581's which are really a KT66 with very good results, I just played them last night for break in.. ran them 2 hours at very taxing volume with those tubes and no issues..
6v6's sound good, but seem a little weak..
J
What is the voltage output of the device you are attaching to this amp? Reason I ask, is some of these organ units had pre-amps (with tone and volume controls) prior to the main amp. If you have a unit that has more voltage output, maybe the amp will work better.
Guys that resurrect PA and organ amps for guitar amp use sometimes need a pre-amp box that puts out voltage-range output, for the vintage PA or organ amp to work okay.
Just a thought...
Fender,
right now since I am "protoyping" the amp I am using my home audio pre-amp Scott 121..I have not measured it's voltage output..but the end goal is for me to have this set up as a guitar amp for my friend. I am not a guitar player so I am not sure sure if the voltage out put is similar to my pre-amp. I was going to have him come over and plug in. I suppose I can measure the output or the spec might be listed for my pre.. and compare to his output.. but yeah you bring up a good point..
Right now I have to crank my pre to 11 o'clock before I get good dynamics and then to 1:00 before I reach full safe volume..
I have 3 of these, I was hoping to give one to my friend for a practice amp, and the other two, use for home audio. I was going to use them as loaners, when I work on other folks amps, or for sale if I need money for a project...
thanks for the input...
J
-Then I doubt you need any additional gain.
This is because you will likely have plenty with whatever will be driving this, and most guitar speakers are really easy to drive!
hey thanks for the input. I am going to bring home a guitar today and pipe it in and see what we get. Very good point on the guitar speaker, he is building the cabinet, and using a very efficient 8 OHM guitar speaker from eminence I believe..
can't wait to try it..
Like some OD pedals. Try it out and see if it helps the amp's gain stage.
will do! if we just had a bit more gain, this thing would rock the house.. I plugged in a guitar straight into the amp tonight and it sounds amazingly pure, the pickups just don't have enough to drive the amp to full potential...it's maybe a 1/3 of the way there...
we might have some pedals at work in the sound room I can try..
J
One way you can increase gain is to decrease the amount of global feedback by decreasing the value of R3, or to get rid of NFB altogether. This approach will work fine if you are using antique style speakers that do not need much electric damping. Prior to the Hi-Fi era, they did not use global NFB in pentode amplifiers, even in high end products like Magnavox Regency or Belvedere consoles, and these radios sounded fantastic. Of course, if you are using modern compact low efficiency speakers, you will need at least some NFB. You can play with R3 and find the lowest amount of NFB you can live with.
If you want to add an RC gain stage, you can use 6J5, which is a single octal triode with characteristics similar to 6SN7. Use the following part values:
Rk=2.7k
Rp=100K
Coupling capacitor 0.05
Rg=470K, volume control pot with log taper.
B+ can be obtained from the following stage through a dropping resistor, so that plate voltage is about 180V. B+ should be bypassed to ground with 20 uF electrolytic capacitor.
With 6J5 Rk bypassed with 50 uF capacitor, gain will be ~15; without capacitor it will be ~5.
The simplest means of raising the gain would be to replace the 6SN7 with a 6SL7. It's not plug-n-play; you'll need to change all the resistor values around the preamp and phase splitter. Still easier than adding stages or changing tube sockets.
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Buy Chinese. Bury freedom.
You know after testing with an actual guitar this may be all I need to do is go to the 6sl7, I thought I would just have to change the plate/cathode resistors to I am guessing higher values as the Ma rating for the 6sl7 is much lower..
Is this all I need to do and would you increase these values by how much ?
No, the circuit you posted can't be driven directly from a guitar, even with a 6SL7. The 'SL7 will only bring its gain up to what is required for a line level amplifier (~1V RMS input). If your intent is to build a guitar amplifier, you'll need additional preamp stages.
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Buy Chinese. Bury freedom.
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