Leslie Combo Pre III & Leslie 25 Issues.

[MangoHands]

Hello All.

Its been a while since I've posted on this board.

Anyways, I've got a Leslie Combo Preamp III and a Leslie 25.

Anytime I plug something into the combo pre and play it comes out very low volume and very distorted.

The other day I was messing around with it and saw sparks fly inside one of the 6L6's.

I replaced the original speaker with an 8ohm 50 watt 12inch woofer from a passive PA cab. I modified the booster amp in the Leslie 25 to accept 1/4 inch speaker plug.

I'm wondering if it has something to do with the speaker wattage, or if its just a bad tube, possibly.



FWIW: I recently had the preamp serviced. The guy told me he had to replace some old transistors (???)

Anyways, if this has ever happened to anyone or if they know of a possible solution to my problem that would be most wonderful.

Thankyou.

[picothinker]

I'm not familiar with this Leslie, but most of them from this era had 16-ohm drivers. Captain Foldback's site mentions that it originally had a 20-watt amp. A higher wattage speaker than the amp does no harm, but an incorrect impedance speaker can.

[jimmywilliams]

Low distorted sound from a Leslie 25 with a combo preamp usually indicates that the 8-ohm load resistor on the 25 amp is engaged. You don't want that. If it is an older model it might have the load resistor as a ceramic "smokestack" resistor in the switching junction box. Pull it out of the socket. If it is a newer model it will have a jumper plug on the top of the amp. Unplug it.
Not sure how you have a CPA111 hooked up to a Leslie 25. The cpa111 is 11-pin but the 25 is a 6-pin. Are you using some sort of adapter?
Picothinker, the 25 has an 8-ohm driver, as do similar models like 110, 125, etc.

[picothinker]

Ah, I stand corrected! Thanks.

[jdoc]

that is what I thought, Type 3 is for 860 and 130 etc........

Low distorted sound from a Leslie 25 with a combo preamp usually indicates that the 8-ohm load resistor on the 25 amp is engaged. You don't want that. If it is an older model it might have the load resistor as a ceramic "smokestack" resistor in the switching junction box. Pull it out of the socket. If it is a newer model it will have a jumper plug on the top of the amp. Unplug it.
Not sure how you have a CPA111 hooked up to a Leslie 25. The cpa111 is 11-pin but the 25 is a 6-pin. Are you using some sort of adapter?
Picothinker, the 25 has an 8-ohm driver, as do similar models like 110, 125, etc.

[MangoHands]

I'm sorry. I misspoke about the combo preamp. I have a combo pre III for my 860.

I have an original CPA that's 6 pins that Im using for the 25...

Anyways, you're saying if there's a white smokestack resistor on the switching junction box I should take it out?

I've attached a photo. If you could point out which one (as there are 3) I'd need to pull I'd be very grateful.

errr... Here's a craigslist link with the picture of mine in it.... (Can't figure out how to attach photos)

http://chicago.craigslist.org/chc/msg/3737654950.html

[jimmywilliams]

Look at the photo and notice there are two parts - there is the junction box that the 6-pin Leslie cable attaches to from your preamp. This junction box controls the motor switching and also sends signal to the amplifier itself (the larger chassis) with the tubes.
On the small junction box you will see the 8-ohm load resistor up front, right near where your 6-pin cable frim the preamp plugs in. It is mounted in a black plug so it can easily be removed from the 2-pin socket. It should be marked elect-ohm 8 or something like that.
Do not muck around with anything else on that small chassis. There are also two very tall smokestack resistors in the back. These are for the braking voltage and should not be touched. Those are marked with a higher resistance value and are not mounted in the black plugs, so you would not be able to easily remove them.
Does this explain better?
Not feeling well so I may check back in a few days.

[MangoHands]

Many thanks!

[jimmywilliams]

Let us know how it goes with the preamp after removing the load resistor.
Also do not discard that resistor. You will need it plugged in if ever tapping off the speaker output from an organ like the standard 26-1 hookup kits are designed to. But for a combo preamp hookup, it must be pulled out.

[MangoHands]

Hey guys.

So, turns out when I removed the load resistor, nothing happened. No signal passed, nothing.

I started to notice an odor coming from the booster amp though. The other day I saw sparks fly in one of my tubes. I'm wondering if this has anything to do with it.

Maybe replacing the tubes?

I'm at a loss as to what I should do here....

[Wes]

I doubt the load resistor will load down the output of the CPA too much, it should work either way. Just quieter with the resistor in place.

Read about servicing tube amps safely. Be careful, they can kill you.

Get the amp and schematics out. Measure the voltages under the hood. If they are wrong, figure out why. The 25 amp is VERY simple. But still deadly.

Suspect the two smokestack resistors in the amp. They are not for the braking circuit, that is handled in the motor control box by/near the light bulb. Those resistors set some of the voltages inside that amp. I can't tell you what for because my tube theory is weak, but I assure you, they are critical and have nothing to do with the brake.

You might be able to swap tubes from a guitar amp for testing, it couldn't hurt; you need a pair of 6L6s, a 12AU7 and a 12AX7 if my memory serves me correctly.

As long as you are servicing this amplifier, you should replace the multisection filter cap if is more than 20 years old. You can buy a new one from Tonewheelgeneral.com.

Wes

[jimmywilliams]

Just to clarify a few things ... Re the smokestack resistors I was only talking about the motor switching junction box and not the amp. I Just looked at my switching box and in the back are two 50-ohm 20w resistors. Seems to me like they are there to limit current for the braking voltage since engaging the relay will put them in series with the brake coil - at least that is what I can make of it. My box does not have a current limiting light bulb (different revisions of this box were made).

Taking the load resistor out of circuit should not cut all signal to the amp since it is in parallel not series. I just verified this by looking at the wiring in the junction box. If you say you stopped getting any signal after you pulled the load resistor out of the 2-pin socket that does not sound right at all.

Keeping the load resistor in circuit while only using a preamp will indeed cut the volume and distort the signal - I had it happen to me personally. Pulling it out will present a higher impedance to the preamp output which is what you want.

Two things alarm me -

The obvious first is that there is any sort of burning/sparking/etc. Stop using this until it is repaired. Take it to a pro if you must. It is not safe. If you are comfortable with this type of work then take Wes's advice and be careful.

The second is that iirc you said something about a different type of input jack somehow wired up in the amp itself? Is this true? Please clarify as this would indicate some mod was made to the amp and that can be trouble if done incorrectly.

[MangoHands]

Hey Guys,

So I took my speaker, amp, cable, and preamp to a pro here in Chicago.

Turns out I shorted out some caps in the amp (.001 @ 16k vlts & .1 @ 600 vlts) and in the process, it popped my 6L6s and 12au7.

Anyone know where I can grab those caps at a decent price?

They wanted to charge me close to 200 bucks for everything. I refuse to pay that since I know my way around a soldering iron...

Any help would be much appreciated!

[Wes]

Wait till you find out what the tubes cost....what did they charge you for the diagnosis?

Those caps are available at the usual sources - digikey, mouser, Newark, etc.

Wes