tone controls on 5E8A low powered tweed twin

[phsyconoodler]

I have a replica low-powered tweed twin in for some speakers and the tone controls are weird.
  You turn the bass past about 7 and the amp gets lower volume and almost cuts out.You turn both the treble and bass all the way up and the amp oscillates and sounds like total crap.Some guy says it's normal on another site.
  Anyone know if this is normal? If so,it sucks!

[g-man]

Deleted

[printer2]

The first tone controls in amps were sometimes a little funny. The 5E8 had a Tweed tone control for the treble and they added a bass control that had quirks. Download Duncan's tone stack calculator http://www.duncanamps.com/tsc/ and check out the E series tab.

If you really do not like the tone controls you have you could always replace it with Fender's tone stack that is used till today. Ignore the PI section of the amp, after the second stage cathode follower you will see a DPDT switch to use either the E series or current tone stack. I drew it up to answer someone's question between the Bandmaster and Bassman, the tone control section shows you can replace yours. Since you only have holes for bass and treble you would use a resistor for the mid control.

[stratele52]

Not normal for sure , many wiring error IMO

[jjasilli]

Maybe checkout:  http://geofex.com/ > -Tube Amp Debug Page > Low Power or Loss of Volume

[phsyconoodler]

Thanks guys.I knew it wasn't totally normal.This amp has original old Astron coupling caps in it and I'm thinking they are bad or at least one or two are.
  The wiring is impecable but I don't like the way these work and the amp isn't loud enough.I'm going through it and finding out why so I'll keep everyone posted.

[tubeswell]

Is the pre-amp's power supply decoupling cap okay? (I'm wondering this because you mention how the volume is affected with varying frequency in the tone stack, and so I'm pondering whether the changing AC loads are causing unwanted feedback between preamp triodes to an improperly decoupled plate supply in the power rail?) Just a long-shot on my part.

[HotBluePlates]

No, I'd tend to believe the volume bit is normal.

I didn't have an electric handy recently when I was in the same room as my tweed Super copy (I'm in another state now). This amp has the same tone circuit as the amp in question. Last I tinkered with the controls, it seemed that you could easily lose all volume when the tone controls were set to full zero.

As Printer said, if you use the "E Series" tab in the Duncan Tone Stack Calculator, you will have a sim of this circuit. It will show almost a complete volume loss with Bass on 0 and Treble on about half-to-1.

With both controls full-up, there is very very little loss except for some mid-scoop; obviously it shouldn't oscillate, but that tells you something about the behavior vs signal strength at or just-after the tone circuit (or in connecting wiring).

[jjasilli]

Hotblue:  Psycho's first post - "You turn the bass past about 7 and the amp gets lower volume and almost cuts out.You turn both the treble and bass all the way up and the amp oscillates and sounds like total crap.Some guy says it's normal on another site." 

It seems the issues are when the controls are turned UP, not down.

[HotBluePlates]

It seems the issues are when the controls are turned UP, not down.

  Thanks! I must need more sleep! 

[tubeswell]

The 5E6A circuit also has that quirky local feed back around the DC-coupled pair driving the tone stack. Is that all wired correctly?

[Fresh_Start]

My first suspect would be that .005 cap shared by the bass and treble controls. How difficult would it be to wire a new, separate cap there for the bass?  I'm sure it's crowded in there...

Cheers,

Chip

[phsyconoodler]

The builder used original astron caps throughout the amp and I checked them in the circuit one at a time.
As it turned out,every single astron cap in the amp leaked dc. I replaced them all and the amp is 100% ok and sounds awesome.
  Go figure that every astron was leaking,not one or two but all of them!!!

[g-man]

Deleted

[Platefire]

So the owner brought it to you for speaker installation. Did the owner think the bad sound was due to the speakers or did he even know anything was wrong with the circuit? Sound like a lot of my jobs, they bring it in for one thing and I end up working on something else.  Plate

[phsyconoodler]

He just bought the amp and it had modern Fender lightning speakers in it.They sounded ok but the amp wasn't loud enough and the tone control issue I discovered after changing the speakers out.
  He is a tone nut and wanted the amp to be awesome sounding but it just flat out wasn't.
 It is now,that's for sure!
   The bass control has a very small volume drop out when maxed and all the oscillation is gone when maxing out the treble and bass pots.Actually it's not really a volume drop,it's a frequency change or shift.
  No one ever playes with the controls maxed out but it's good to know how the work.

[HotBluePlates]

... As it turned out,every single astron cap in the amp leaked dc. ... Go figure that every astron was leaking,not one or two but all of them!!!

If I remember right, Astrons are a paper dielectric cap, encased in wax.

I assume the manufacturers used wax back in the day to keep moisture out. And it does, for a while. But on a time horizon of 30, 40, 50 years, some waxes will absorb moisture to a great enough degree to be a problem. This is part of the reason Fender's waxed fiberboards sometimes develop problems.

It is interesting to note that radio restoration guys universally assume any old paper-dielectric/wax caps are bad. This include Sprague bumblebee caps among others. Bottom line, these guys noticed that all these caps eventually go bad, so rather than test each cap in a radio, they yank out all of them and replace ("... because if it hasn't gone bad yet, it will tomorrow").

So it doesn't surprise me all the Astrons are bad (I once had a '54 Princeton that had all of the Astrons replaced with non-leaky caps; the seller put the bad leaky caps in a bag for the buyer if it mattered to them). Too bad you probably can't do what the radio guys do and encase modern caps in the old Astron bodies. The radio guys do it so the innards have the nostalgic look, not because any of their customers will pay more for old caps. I suspect that some of the modern "vanity brand" caps use such a technique rather than being anything special inside.

[PRR]

> used wax back in the day to keep moisture out

Also to get more uFd.

Plain paper has low dielectric constant, partly because there is a lot of empty (air) in it. 

Wax has much higher dielectric constant. And fills the space in the paper. And keeps moisture out for many months (longer than the warranty). And nothing exotic or single-source. Does the job, good enough, cheap.

The purer waxes ought to last a long time. I suspect they cut the wax with whatever to save a penny, and then thinned it with solvent so it would soak-in faster.