Tweed 5E3 Deluxe Project Problems

[Glowin Plates]

Hi All

I am having a few problems with the Tweed deluxe Model 5E3 amp I am building.
I have the amp working, signal is getting through and it has volume but if I have the tone control anywhere but all the way down the amp breaks into wild squeals and oscillations.
I have adhered to the standard schematic (attached) with the following exceptions.
I used a 12AT7 instead of a 12AY7 as listed in the schematic.
I know this is a higher gain tube, should I adjust any components for this?
I used a 820Ω resistor for the cathode and 100KΩ for plate.
I only used 1 side of the 12AT7 as I only need 1 input.
The schematic did not list a value for the coupling cap between pins 6 & 2 of the 12AX7 so I used a .1 ufd as elsewhere in cct.
I have double checked the circuit many times but it's possible I have
mis-interpreted something or just done something wrong.
Any ideas what would cause the squeals for starters?

[singa]

Hi, The squeals mean that the amp is oscillating (not good).Since you have
used a tube that the crcuit is not designed for,if there is feedback from the output then try to reduce the value of this resistor.

On the other hand the tone control may also contribute to the
oscillation because it is a feedback circuit anyawy but there is a way to add a resistor and a small value capacitor to move the away from the resonating frequency but I don't know how to calculate it got to refer to tube theory books. singa

[Blind Lemon]

Try swapping the blue and brown wires to the plates of the 2 6V6's. The 12AT7 shouldn't make a difference. I have run 12AX7s in mine with no problems. If the plate wiring change doesn't help go over all your wiring again. You might try posting a pic of the guts.

BL

[Trout]

Originally Posted by Glowin Plates

Hi All

I am having a few problems with the Tweed deluxe Model 5E3 amp I am building.
I have the amp working, signal is getting through and it has volume but if I have the tone control anywhere but all the way down the amp breaks into wild squeals and oscillations.
I have adhered to the standard schematic (attached) with the following exceptions.
I used a 12AT7 instead of a 12AY7 as listed in the schematic.
I know this is a higher gain tube, should I adjust any components for this?
I used a 820Ω resistor for the cathode and 100KΩ for plate.
I only used 1 side of the 12AT7 as I only need 1 input.
The schematic did not list a value for the coupling cap between pins 6 & 2 of the 12AX7 so I used a .1 ufd as elsewhere in cct.
I have double checked the circuit many times but it's possible I have
mis-interpreted something or just done something wrong.
Any ideas what would cause the squeals for starters?

That particular cap is .022 on the standard 5E3. Shown on the standard layout


As mentioned above, 90% of the time the squeal is simply reversed output transformer primary leads.

Occasionally guys forget to ground the output jack when they use the isolated cliff jacks. That will also cause a severe squeal.

[Glowin Plates]

Reversing the leads to the plates of the 6V6's had no effect on the problem.
I notice also it has a pretty severe hum with no guitar plugged in.

[M Gregg]

Just a thought,

Try pulling the first tube 12at7 and see if it still responds with the change in tone control! OK you won't get any sound out of it..LOL.

This will give you an idea if its from the 12at7. The B+ should be blocked by the 0.1 coupling caps. If you have no squeel then you need to look at the biasing of the 12at7. You could try changing the 820 on cathode to 1.5K. It could be microphonics from the tube and the level of gain.

Regards
M. Gregg

[Trout]

Originally Posted by Glowin Plates

Reversing the leads to the plates of the 6V6's had no effect on the problem.
I notice also it has a pretty severe hum with no guitar plugged in.

Is the output jack properly grounded?

The Hum can be from multiple problems.

If all of the parts used are of standard values, (resistors,caps, tubes) you should not have to change anything. There are hundreds if not 10's of thousands of guys that have built these from scratch without any circuit mods needed.

Start by posting a few good pictures of the wiring/internal chassis.
Other specifics like layout, transformers used would be helpful as well.

Also try to get actual voltage readings at all three power supply nodes & at the tubes.

[Blind Lemon]

Do the easy stuff first, suspect the tubes. Replace the 2 preamp tubes with known good ones. Halving the the value of the cathode riz shouldn't make that big of difference, but I will say that is is better to build to spec first and then start moding. The original layout and values have been working since '55. Looking at the layout the .1 coupling cap is correct. As far as gain goes, like I said above, I can run anything from an AU7 to an AX7 in V1 in mine and it will remain stable. The AY7 is hard to beat all though if I want a little more grind I will use a 5751. About the only place I use the AT7 in guitar amps is in the PI and the Reverb driver. Oh and check ALL your solder joints. Is this a cratch built or a Kit?

[jjman]

Since a 5e3 does not have NFB, switching the primary leads on the OPT will not create/resolve (P)FB induced oscillation. 12at7 is generally considered a no-no in gain stages in guitar amps. I don't think any classic or production models use them that way. Higher value coupling caps might increase the risk of "farting" purported to be an issue with this circuit.

I would start with the stock circuit values/components and see if any issues remain or if tweaking is needed from there.

[Glowin Plates]

Thanks For the ideas.
I'll have to pick it up in three weeks when I'm back from Spain.