Converting 6V6, 6cg7 & 5y3 phono amp into guitar amp...Is my PT Bad??

[skydogda]

Ok, this is my first 'bulid', I ordered a RCA phono amp with 6cg7 (2), 6V6 (2) and a 5y3 rectifier. It was stated as 'working' when purchased and I simply wanted to add a TS input jack and use it for a guitar amp.
The sound is barely audile coming from both the TS input and the RCA input jack when tested. After replacing some cap's and reading more about tube amps, I tested the heater filament supply to the rectifier with 2 and 8 pins being the heater pins. The required voltage is 5 v and I got 5.5 for 8 and 0 v for 2. Isn't both pins suppose to be supplied with around 5 v?
Just by examining it with the tubes 'on' I could see them glowing but they weren't bright and only one side was working. I tried to run the 5.5 v from pin 8 to pin 2 but that didn't work.
So, is there a way i can run 5 v into pin 2 from somewhere...Or should I just look for a new PT? However, looking at PT's usually it seem like they only have two secondary windings for filaments, a 5v and a 6.3v. Im confused, any information on this subject would be greatly appreciated. Thanks!!!

[tubeswell]

If one 'side' of the tubes isn't heating up, its because either your non-glowing tube is bad, or you have it wired wrong. It is not your PT 5V winding.

Each PT winding is just a length of wire wrapped around the iron core of the transformer. If one side of it works, then the other side of it will 'work'. Don't short the two sides of the winding together though.

To test the PT, unplug all your tubes and (with the amp off and unplugged from the wall and the filter caps discharged) disconnect the B+ wire running to the first filter cap.

Then measure the VAC from each end of each winding to ground.

You should get:

A high VAC on each side of the high-tension winding (something around high 200s to low 300s for a typical 6V6 amp)

5VAC on the rectifier winding

6.3VAC on the heater winding

If you do, then the PT is not likely to be a problem

[skydogda]

thanks for the fast response...ok, when I tested the 5 v winding going to the second pin I actually tested the exposed wire directly and didn't get a voltage without any tubes installed for the winding leading to the 2 pin. I watched the rectifier tube being tested so I know that its good. But both sides of the heater should be getting supplied with the 5 volts right? Most trannies I have looked at only supply a 5 v winding and a 6.3 v winding, not 2 5v windings.


-Ok, I will go check tonight and let know you. Thanks!

[tubeswell]

Okay just to check that we are on the same page - each winding has two ends - one end of the 5V winding goes to pin 2, the other end of the same 5V winding goes to pin 8. One end of the HT winding goes to pin 4 and the other end goes to pin 6. Measure each pin in AC volts on your meter, with one clip of your meter going to each respective socket pin and the other clip going to the chassis (ground)

Your PT also should have a 6.3V winding (- does it?)

[skydogda]

Ok, I understand now...I will test all the pins again tonight and post results. Maybe the other end of the winding is messed up? Last night I couldn't get any voltage reading on any setting of the 2 pin leading in or going out. Also, another question about rectifiers; all power is rectified thr the 5y3 before leading out to other tubes and components? Or does some of the AC current go to other parts? And how do I test the DC voltage coming out of the rectifier?
Yes it does have a 6.3 v winding but measures around 6.7 v...

[tubeswell]

The rectifier takes the high voltage AC from your PT High-tension winding and puts all the 'voltage swings' on the same side of the ground potential. Then when you connect a capacitor filter to Pin 8 of the rectifier, that smooths out the 'bumps' in the rectified voltage leaving a high DC potential, which is your B+. This high-voltage DC is used to power the plates, thereby drawing current through the tubes from the cathodes.

However if the cathodes (which are made of an alloy that 'gives up' electrons relatively easily when heated) are stone cold, they will not be very forthcoming with electrons. So a separate current (in this case via the 6.3VAC filament winding on your PT) (but in cases where the heater filaments are wired in series, it is a VDC supply) is supplied to heater filaments that are right next to the cathodes (of your pre-amp and power amp tubes), so that the cathodes get nice and hot (orange glow) so that they can more-easily give up electrons, which will then flow through the tube to the plates (which remember, have been raised to the high DC voltage).

When you twang your guitar string, that induces a tiny (few mV) AC voltage via your guitar pickups which goes, via your guitar cable into the amp. This tiny mVAC swing hits the signal (control) grid of the first stage. This signal grid in a single gain stage of your typical pre-amp tube, is situated between the cathode and the plate. Since there is a constant current flowing through the tube from the cathode to the plate, when you insert a tiny mVAC swing from your signal grid into the current, it causes a corresponding VAC swing to appear in the current flowing between the cathode and the plate. Since the plate is at a higher voltage DC than the cathode or the signal grid, then the corresponding voltage swing at the plate is bigger than the voltage swing at the signal grid. This is how the signal is amplified.

However, since you can't have a high DC going into the signal grid of the next stage (otherwise the whole concept wouldn't work), you need to block the DC and only let the AC pass through from the now-amplified plate voltage swing, into the signal grid of the next stage. So you have a coupling (or blocking) capacitor between the plate of the previous stage and the grid of the next stage. In this way the signal is gradually amplified as it goes through the subsequent gain stages.

That is (simply) how DC and AC work together in the amp. You need high voltage DC going to the plates, but you need and AC signal swing at each grid.

If your heater filaments in this case aren't wired correctly to get 6.3VAC, you might not have enough heat to get the electrons flowing properly. The heaters in this case need o be wired in parallel. The best way to do this is to have one side of the 6.3VAC winding going to pin 2 of each 6V6 in turn, then to pin 9 of each 6CG7 in turn, then have the other side of the 6.3VAC winding going to pin 7 of each 6V6 in turn, and then to pins 4 and 5 (wired together) of each 6CG7 in turn. If the the 6.3VAC wining has a centre tap, then run this to the chassis ground return point along with the other power amp ground returns. Otherwise if the 6.3VAC winding doesn't have a centre tap, then you can make an artificial centre tap by running a 100 Ohm resistor from each side of the 6.3VAC filament winding to the chassis ground return point along with the other power amp ground returns.

FYI the pin out of a 6CG7 is here:

http://tdsl.duncanamps.com/show.php?des=6CG7

The pinout of a 6V6 is here

http://tdsl.duncanamps.com/show.php?des=6v6

and a 5Y3GT

http://tdsl.duncanamps.com/show.php?des=5Y3GT

[skydogda]

Wow, thanks for all the great information...that really helps me understand tube amps, esp guitar tube amps now. Ok, so I tested all the windings:
Windings
w1: 335 v {pin 4 5y3}
w2: 335 v {pin 6 5y3}
w3: 3.5 v {pin 8 5y3}
w4: 10 v {pin 2 6V6}
w5: 6.3 v {pin 7 6V6}
w6: 5.4 v {pin 2 5y3}
w7: 0 v {pin 7 6V6} (however didn't disconnect from tube socket)
--strangely enough the windings that didn't have power when connected to the tube sockets had power after disconnecting them...)

I am going to desolder the few remaining windings and test again, bc values would range from 5.7 v when connected (pin 8 5y3) to 3.5 v when disconnected and from 7.1 v (pin 2 6v6) to 10 v disconnected and 0 v (pin 7 6V6) to 6.3 v disconnected and etc...why does this happen?

I need to find out how to fix this probelm before I can run the filament VAC's.
Thanks

[tubeswell]

Ok, so I tested all the windings:
Windings
w1: 335 v {pin 4 5y3}
w2: 335 v {pin 6 5y3}
w3: 3.5 v {pin 8 5y3}
w4: 10 v {pin 2 6V6}
w5: 6.3 v {pin 7 6V6}
w6: 5.4 v {pin 2 5y3}
w7: 0 v {pin 7 6V6} (however didn't disconnect from tube socket)
--strangely enough the windings that didn't have power when connected to the tube sockets had power after disconnecting them...)

The HT winding puts out 335VAC each side. So with a 5Y3 you should get a B+ around 370VDC, which will drop a bit (to 350-360) under load (i.e.; when you have all the tubes plugged in)

The fact that you have such a low voltage on pin8 of the 5Y3 (and a slightly higher-than-normal-voltage on corresponding pin 2) makes me suspect that your first filter cap is no good (possibly shorted) (which is a possibility with an old unit). If you have Pin 8 disconnected/de-soldered from the B+/ first filter cap node, does the reading come up to about 5V? (Electrolytic filter caps don't last very long compared to other capacitors, so all the e-caps probably should be replaced with new caps of similar/equivalent (capacitance and voltage-rated) values anyway.

[skydogda]

Ok, so I have determined that the filter cap is bad, whenever I connect the filter cap wire to pin 8 5y3 the VAC drops to 0, (but it still reads low around 3.5 v) . I have heard somewhere that instead of buying a new filter cap, you could replace each cap and drill a hole ground it to the chassis. Is this true?

[tubeswell]

Ok, so I have determined that the filter cap is bad, whenever I connect the filter cap wire to pin 8 5y3 the VAC drops to 0, (but it still reads low around 3.5 v) . I have heard somewhere that instead of buying a new filter cap, you could replace each cap and drill a hole ground it to the chassis. Is this true?

Electrolytic caps age faster than other caps, because of the way they are made. Useful life in an amp is about 10 or so years. If you have a 50-year old tube amp, chances are the electrolytic caps have never been changed, and chances are that in order for it to have any chance of working, you have to replace them with new ones (throughout the amp - everywhere that there are electrolytic caps - including those little cathode bypass caps). The filter caps need to be rated to take the peak voltages that the tubes produce under load, which in a PP 2 x 6V6 amp is about a 450V - 500V rating. The cathode bypass caps in a typical 5E3 style amp only need to be rated at about 6V for the pre-amp tubes, and 25V for the 6V6s.

[skydogda]

Ok, I have removed the old filter capacitor can (after lots of twisting and pulling). Now what? I thought that I could ripped the 'guts' out and reinstall the can and externally out the capacitors, however I can not get inside the can. Can I simply add the capacitors to the same post without removing the internal filter cap's? I was thinking that it would also short those out since it's shorted.

[tubeswell]

Yep you wire the new caps in so that the -ve ends are going to the chassis/ground return path, and the positive ends go to which ever respective node on the B+ line they need to go to. You might want to mount the new caps on terminal tag strips/mounting brackets or on a turret/eyelet board set off from the chassis with some spacer nuts, and use cable ties or somesuch thing to secure them to the bracket/board

[skydogda]

Good deal...I will look into getting a turrett/eyelet board for the nodes. My power transformer, however is bad as well. I have disconnected all the windings and discovered still some are extremely high and some are extremely low and also accidently let the wires touch the chassis (ground) and heard them sizzle. Looking on ebay i found one with the 6.3 and 5 v windings, however the plate v's are set for 240. Will this be high enough voltage for the plates to attract all the e's?

[tubeswell]

Good deal...I will look into getting a turrett/eyelet board for the nodes. My power transformer, however is bad as well. I have disconnected all the windings and discovered still some are extremely high and some are extremely low and also accidently let the wires touch the chassis (ground) and heard them sizzle. Looking on ebay i found one with the 6.3 and 5 v windings, however the plate v's are set for 240. Will this be high enough voltage for the plates to attract all the e's?

With 240-0-240 you will need to use Full-wave diode rectification to get around 320-340VDC, so the 5V winding wouldn't need to be used

If it was 240-240 (without a Centre Tap) you would need Full Wave bridge diode recification to get around 320-340

FWIW You can make an eyelet board cheaply with a bit of bread board and some smallish brass eyelets, and mount it using stand-off/spacer nuts. I make my own eyelets boards out of electrical switchboard this way for about $5 per board.

[skydogda]

Ok, continuing the process of this conversion. I have ordered a tranny out of an amp that used a 6V6 and 5y3 and successfully installed it. Everything is done and all is left is to hook up the tranny, however this is where I run into probelms. Availble is the color coding for the wires (http://www.nostalgiaair.org/PagesByM...3/M0008183.pdf)
, however the wires are so old that I can barely identify the wires i.e.

http://www.auctiva.com/hostedimages/...0,0,0&format=0.

My question is how do I figure out which wire is ground and which would be the B+. After that I can figure out the others via multimeter I already ruined one tranny and at this point would probably smash the amp if I had to get another.

Any help would be wonderful, I am stuck between a rock and hard place. Many thanks!!!