Old tranny...low heater voltage

[guitardude57]

Howdy,  I am putting together a build using an old Hammond PT.
I don't remember which organ it was, as I have collected some different trannies over the years.

Hooked to power, unloaded... is showing as a 320-0-320 with 5V recto taps
and heaters with a center tap.

I am getting a solid 5 volts on the recto tap.
5.2 volts on the heater...hmmm too low.  This is with or without, the center tap grounded.

Since this is being used with SS rectifier, and I am not using the 5 volt tap...
Strapping the 5 V with the heater in series, I am getting 11.95 volts, with the heater tap to ground.

So... I either need to figure out how to get the 5.2 up to 6.3, or do a 12.6 volt heater set up, with 11.95 V.


Thoughts?


Thank ye muchly

[Paul1453]

Just a thought.   

Maybe go DCV for your heaters?

Rectifying your 5.2VAC would give you a little over 7VDC.

After some filtering maybe you could get your 6.3VDC from it?   

[guitardude57]

I read up a bunch on DC heaters...and it would seem there are some benefits...and not.
I believe the current (unknown) may be good, since the wire for the heaters is solid core and 14 gauge...

Still, not opposed to 12 Volt either for that matter...in AC

[sluckey]

Still, not opposed to 12 Volt either for that matter...in AC

Something to consider... A 5V winding is usually rated for 2 or 3 amps, occasionally higher. Knowing the current rating of winding your PT winding would be helpful. My point is that if your amp will require 12.6 volts and 4 amps of filament current but your 5 volt winding is only rated for 2 amps, then that winding will be stressed.

[jjasilli]

Just to be sure;  you're getting 5.2V outer leg -to- outer leg?  2.6V CT -to- ea outer leg?


Note 5.2V should be sufficient to run 6V heaters, but the voltage may drop below 5V under load.


Ditto to Paul 1453. BUT the low voltage suggests the possibility of a partial short in the heater winding or leads, bad insulation, etc.  You might want to poke around inside a bit to see if there's a fix.

[guitardude57]

Thanks for replies guys...

Yes the tranny has 2.6V leg to CT, both sides, each.

CT to each leg resistance .12R
Leg to leg .16R
So, that looks to me as no short in winding.

Both the 5V and the 6V are 14 ga solid wire.
The original chassis IIRC, had 7 tubes... so I believe the current to be at least a couple amps.

[eleventeen]

"So, that looks to me as no short in winding."


You really can't be sure from your reading...unless you have REL'ed out your meter...if it has that feature. When we get down to .1 or .2 ohms, we're generally looking at the resistance of your test leads. Not saying it's one way or the other, just that you can't really conclude either way.


You know...it's pretty rare, but the 5 volt winding on a PT assuming it ran a 5Y3 or 5U4 etc; carries the full B+ of the power supply. It's not just 5 volts. Thus it's definitely not immune from some kind of arcing short from its many years of service.

[guitardude57]

I have a good Beckman (Med Grade) and a Fluke DMM's... so very low reading are accurate.

Anywho, I rigged another transformer of appropriate size and function, since I am not trusting the other.

The wires coming out, are in fine shape and supple.  The other tranny, the wires were pretty hard.

The current one being tested is an AO 24110-1 Hammond

With 118.83 VAC from wall:

Getting 5.76 V on the 5 Volt...

7.26 V between the heaters (CT) feel good about that, unloaded.
CT to leg 1  3.63 V
CT to leg 2  3.63 V
 Resistance CT to leg 1 is .15R and same to leg 2.  Leg to leg  .22R   (for references)


HT CT to leg 1  348.2 V   Resistance 78.50R

HT CT to leg 2  348.8 V    Resistance 85.75R

Leg to leg  693.0 V   Resistance 164.16R

This one is working...

[sluckey]

Much better! This PT will really simplify your build. Was this salvaged from an organ amp? Which one?

[guitardude57]

I have picked up several over the years...sometimes most of the chassis, sometimes just a tranny or two.
I believe this one is from an AO-29 organ Amp.

[sluckey]

Which amp are you building? Which output tubes?

[guitardude57]

A 6v6 powered Plexi.  2 switches up...lead and  2 switches down...bass circuit.
I upped the caps to the PI plates to .047, as the lead uses .022, and the bass .1uF.
So the low end should be OK for the lead circuit, and not so farty on the bass circuit.

The CF on V2 will switch between the stock lead at 500pF, and trying a 270pF for the bass.
I am out of 250pF...lol

Tamura HiFi OT  8K  The matching one is still in the original chassis, that has been a test bed amp over the years.
This has been many circuits, and the 1986 bass circuit for 6 years now.  I have used it pretty heavily for gigs.
It sounds amazing for the wacky look of it.

The chassis I am building in, is a Carvin X100B I got off the bay for 30 bucks.
20.5 X 8.25 X 3.  So lot's O Room.

[guitardude57]

Amp is together, as of Sunday.  I have been keeping busy doing a few power supply mods,
bias range tweaks, and the like. 

My last issues are related to some wire placement and interactions.  I guess I was really expecting it, since the board is directly over the preamp tubes.  Stood off, 1 inch above sockets.

I have a bit of hash coming up at 3/4, on the bright volume, (oscillation, with volume drop as it goes to max) and a similar condition on the master.  The normal channel volume goes to max without a whistle, but since the master volume gets hashy near the top as the bright pot does, I get guitar sound, with crackles.

At half master, it is playable.  At half volume both bright and normal, at 1/3 master, it plays
well enough to see that the lead/bass switching works.
At Full master, half normal, sounds fizzy... add bright at any level, becomes stronger fizzy...and then whistles.

I did use shielded cable from the input to V1, and from treble to master volume.
Input was extremely sensitive, so I swapped the 68k V1 grid loads, with 100k's.  It helped.

Pretty sure I have grid wires in close proximity to heater wires to V1.  If I pull V1, the noise subsides.
The Master still gets the whistle though, near 3/4 full up...  Will also check for DC getting past signal caps at V1b and at Master to PI.

Then to see about moving V1 grid wires around, to try to eliminate the oscillation things.  This will require chop sticks, a flashlight and a mirror to see under the board...lol  I did try to segregate the grids from everything,
when the wiring was soldered under the board for the switches...

It is an unconventional build, as far how things are situated, and the first amp I built, that needed tweaking after building.  I guess I was due a brainteaser.


Will post some pics in the next couple days.

[sluckey]

50/50 chance of getting the NFB phase wrong. Swap the OT primary plate leads. Does it get better or worse?

[guitardude57]

Yeah Steve,
Just woke up...doing the coffee thing....

The first thing I will check on a build is NFB, as you say 50/50 there.
Had to reverse them immediately on power up.

But since I didn't have any 6V6's laying around, stuck a couple 5881's I had around...
just to check voltages, and it rang with them. 

I guess stranger things can occur, and they could be out of phase just enough with
the new JJ's in there.

Have you ever had an out of phase NFB, that wasn't just screaming as soon as you turned it on?

[sluckey]

Have you ever had an out of phase NFB, that wasn't just screaming as soon as you turned it on?

Absolutely. I've even seen a problem on the forum where the symptoms were so subtle that the guy didn't even know he had a problem for over a year. It was a gritty sounding Marshall amp and the problem was found when he was tweaking something much later. The only clue was that when playing loud some notes would burst into oscillation. He thought it was just the notes sustaining due to acoustical feedback. He didn't know the amp sounded raspy until he swapped the plate leads.

[guitardude57]

Yes it was a subtle change...swapping them.

Changing them just now, the squeal happened by turning master up and then
the normal half way...

Put them back, and both go to 10 no squeal.

So it seems... it is something about the master, and the bright channel.

I am going to bypass the master volume...see what difference that makes.

Also noticed, that even with the master off...still getting a little bright channel signal going through with all turned down...strange.

[guitardude57]

Bypassed the master...no difference.
It is the bright channel...

[sluckey]

A stock plexi bright preamp is a simple thing. But you have deviated somewhat. Could you post a schematic of your build?

[guitardude57]

The layout is conventional Lead plexi circuit.

The enclosed pic is the lead - bass switch 1
The second switch just turns on/off the cap at V2 pin 3

Master volume is in between the treble wiper, an PI input.

[guitardude57]

Other changes:
Signal caps to Power tube grids are .047's
Bias loads are 100K's

Bypassing preamp to PI input with audio...is clean and working fine...just a lower gain.

[guitardude57]

I have built 5 different Marshall versions with zero problems on start up...this one
has been wacky.  I don't mind some wrestling with an amp, there is always something new to learn.

I am pretty sure that combining cathode connections, and an extra bit of wire to the RCK's
 to both sides of V1 is part of the problem.

Question, what would be the limit in mV dc found on the back side of a cap?
Caps that I have changed either blocked DC well...or almost not at all...in other repairs.

[guitardude57]

Pics of the beast...lol

[guitardude57]

More pics

[guitardude57]

Got everything running well, except the bright volume still goes to cut off
at more than 7/8 up.  I hear a light pop, then volume drops 90% or more
the rest of the turn to end. 

I hear a little DC sounding noise from pot motion
at the end of last 1/8 turn.  Cap is good and new, replaced once, and no change.

At full up, I am reading -1.7 vdc, between cap and pot.
Below the cutoff, no dc entire pot.

And also the plate gets pulled down from 183v to 176v.
When lowered in level below the cutoff, the channel acts correctly, and the plate voltage
goes back to where it is at 183v anywhere else...set on the pot.
Cathode wires are a little long, but the ground reference is good, as is the solder.

I have moved wires as much as can be moved.
The wire from the .0022 to the bright pot is 2 inches long.

The normal volume is great, and with the master on full, and the 2 volumes at 2/3's volume,
it sounds pretty darn good.

[guitardude57]

Figured out, and solved the bright channel pot cutoff issue.

I relocated wires from under the board, that went from the Lead/Bass switch,
to the 2 treble feed caps, (one for bass, one for lead) that switch with other functions.
They were resting against the grid wires...

Not anymore.

This thing is pretty darn loud!

Thanks to you guys for your replies.