Weak sounding Ampeg J12-R

[dude]

I just changed all the power filter caps in a Seventies Ampeg J-12R, before the change the amp sounded bad, motor boating, static, etc.  She sounds good now but the volume is weak IMO for two 7591's.

I checked the power tubes on a tester, no shorts or leaks and tested well into the GOOD, the CU10 tested good too, all three plates.

I didn't change any coupling caps or resistors as the voltages are right on with the schematic, only about 8 volts higher all down the rail.

I have a SE Champ 12 with one 6L6 that's a lot louder.

The tone control is weak sounding too, doesn't get real bright on 10, bass can't be heard too well on lowest setting.

Anything that I might check or is this what you get from a pair 7591s...? A lot less volume than a pair of 6V6's I have in other amps.

al

[terminalgs]

It should be louder than a champ!  because it easy, I'd jumper wire the speaker from another combo to rule that out,  plus swap the PI 12AX7.  then I'd test the OT for a shorted winding (ohm tests, test for shorts,, and ultimately put 6v on the speaker winding and see what shows up on the primary side -- tubes, and speakers disconnected). after that, I'd go for the 7591A's

did you check your voltages?

[dude]

Not the speaker, plays great in another amp.

Not the PI tube, nor the other 12Ax7, don't have a CU10 but my tester checked it out "Good" all three sides, also the 7591s tested very strong too. Have no spares.

I don't know much about testing the OT but I can look that up. Usually with my experience if somethings wrong with the OT, it's usually dead, or making crazy noises....?

The amp lays fine, no hum very quite but weak on volume. The OT is original, can an OT have a short and still play fine, no strange noises just weak...?

Before I look up how to check the OT, any other simple checks.

The amp starts playing at 3 on the dial goes to 5 and then the same all the way to 10.

Thanks, al

[plexi50]

What are your  plate and bias voltages? Did you check the preamp tubes cathode bias voltages?

[dude]

What are your  plate and bias voltages? Did you check the preamp tubes cathode bias voltages?

Plates        365v  pin three of 7571
Screens      345v pin four of 7591

Across cathode bias resistor (142 ohm)  14.25v

14.25/142 = .100 both tubes or .050 amps per tube or 50 mA

365 - 14.25 = 350.75 * .050 = 17.53 watts disapation


If I'm doing the math right, the tubes bias is around 50 mA and plate dissipation is 17.53 watt per tube. If correct is this in a 7591s range? seems high...?

Thanks, al

[tubeswell]

7591s are 19W tubes, so your'e running them at 92%, which shouldn't matter as they are cathode biased. However, they might be old (no matter how they appear on a tester). See if you can lay your hands on some fresh ones, and try that.

[dude]

Yeah, they're old Westinghouse but who knows if my tester is accurate, that's old too, my avatar.

Before I lay the coin down for new 7591s, do you think it could be the OT?

Like I said before no strange noises just low output, real low against my other 6V6 amps, everything else checks out great.

thanks,

al

[HotBluePlates]

... Before I lay the coin down for new 7591s, do you think it could be the OT? ...

An output transformer gets suspected about 20 times more than it is an actual problem. I'd bet money your OT is fine, without even looking at your amp.

Measure the bias voltage across the output tube cathode resistor (yes, do it again for this test, so you have a recent reading). Now unsolder one leg of each coupling cap feeding the output tubes, and measure that resistor's voltage again with the caps disconnected. If it has changed significantly (say 2v or more), you should replace the coupling caps between the phase inverter and output tubes.

If no change, this is where you'd like to have a signal generator. You'd like to apply a guitar-sized signal to the input jacks and measure the a.c. at the output tube grids to see if the amp is driving the output tubes adequately.

[spacelabstudio]

I've heard reports of low volume being fixed specifically on old Ampegs by replacing the OT.  I have no relevant experience to corroborate that.  I'm not sure what could happen to an OT to reduce the volume but otherwise seem ok.  Maybe core magnetization?  Anyway, I'd definitely want to rule everything else out before plunking down $$ for an OT, though.  If you can scrounge or borrow something suitable as an experiment, though, that might be worth it.

[dude]

I'll check the coupling caps, C18 and C17 and replace if necessary. I hear you on the OT and agree. But if the coupling caps check out ok, it could only be the 7591s or maybe the OT?

Usually my experience with weak power tubes is noises, bad distortion and some lack of power but these tubes did test very strong high on the "Good", no leaks.  The amp actually sounds good but low on volume, bias is good, voltages are perfect. Even the distortion isn't bad.

spacelabstudio, chimed in about a history of old Ampeg OT's having this problem?

I'll replaced those coupling caps to the 7591's plates, if that's no good I do have a new OT on hand that I think will work. Some work but no cash outlay for a good set of 7591s, they say the new ones aren't too good, a used good set is $80 or so on flea bay.

The OT I have is 6.5K ohms -primary (plate to plate) made for a pair of 6V6s, do 7591s want to see 6,500 ohms. It's about a 25 watt OT?

I'll get back, thanks again.

[sluckey]

do 7591s want to see 6,500 ohms.

yes

Output tubes have to be a prime suspect. A pair of JJ 7591s is only $35. Another option, rewire the socket for 6V6s. Should be an easy mod if it is wired like the '60s blue diamond amps. My first choice would be a new pair of tubes.

[dude]

do 7591s want to see 6,500 ohms.

yes

Output tubes have to be a prime suspect. A pair of JJ 7591s is only $35. Another option, rewire the socket for 6V6s. Should be an easy mod if it is wired like the '60s blue diamond amps. My first choice would be a new pair of tubes.

This is where the experience comes in (that I lack), sure is easier to replace two tubes than an OT. I did find another forum where some one had this problem with a B15 but there was never a solution posted....they even tested the OT and found nothing off in the readings...

Not my amp, a friends, I just help other musician friends with their amps (real cheap, too cheap but I like fixing stuff, gives me a lot of pleasure making a broken amp sing, being able to change the tone on my amps and built a PTP amp). I did once change a 7591 Ampeg to 6L6s (went back to 7591) 7591s have a nice sound to them, just different than 6V6s or 6L6s plus it a seventies PCB heavy duty bd. Ampeg made, I'd have to destroy the vintage amp, won't do that.

So,I get the message here to try a know new set of 7591s, makes sense. I hear EV makes the best new production 7591's...? I haven changed or tested the coupling caps yet maybe that will do the trick, if not new power tubes.

I'll find the problem and post, thanks,

al

[dude]

Well, looks like this amp is turning into a nightmare.

I got a new set of EH7591A, they look like 6L6s and just as big, too big. Had to move the speaker down a bit, deal with those crazy Ampeg "screws" but got the tubes in with clearance. First off the new tubes were making some crazy noises and one was coming on and off but settled down enough to play a bit, the volume is the same as with the old stock 7591s, weak. It wasn't the tubes my tube tester is good.

All voltages are good, I guess it's time to check out the OT. I did search other forums about Ampeg OT's sounding weak and found some other people that had the same problems.

At this point I'll just replace the OT as I have one here that I know is good unless anyone can lead me somewhere else to look. I posted voltages previously, all match the schematic very close, bias is good at 50mA, max is 80mA.

The coupling caps feeding the 7591's are good, lifted the legs and the cathode bias voltage only dropped .2 volts.

Besides the the OT any suggestions?

The amp was brought to me as it sounded awful, all kinds of static, couldn't even play it, oscillation, squeal, etc. I replaced all the power filter caps and the amp came back to life seems like it was the filters. Could the bad filters cause the OT to go south...? I think the owner might have just let the amp squeal too long trying to get it to play, maybe fired the OT....?

thanks,
al   

[HotBluePlates]

Could the bad filters cause the OT to go south...? I think the owner might have just let the amp squeal too long trying to get it to play, maybe fired the OT....?

I don't know what might make the OT go bad, but it certainly seems reasonable to suspect it at this point.

If you're repairing the amp for someone else, maybe it is worth your while to have a Deluxe Reverb transformer on hand (carefully check mounting hole locations to see if it will fit this amp). You could unsolder the existing OT, tack in the new OT without actually mounting it, and see if the volume issue is solved. If it is, you mount the transformer properly. If not, you have a Deluxe Reverb OT on hand for future testing and/or a future build/repair.

[terminalgs]

My Gemini-I came rewired for 6V6s with  a small Princeton OT.  terrible sounds, no volume,, etc...   I rewired back to 7591's,  and installed a TF110-48-UL from (Triode Elec/Magnetic.) that I had on-hand.  they still list this OT at $30.   they have other 6.6K 20W OTs, and I've used several.  All of the amps they are installed in sound great,  including the Gem with the TF110-48-UL (did not use the UL taps,,).   

[dude]

The OT I have is a Magic Parts TF-110, it's 6.5K , 8 ohm tap. Looks about the same size as the original in the Reverbojet.

The specs in Ruby's catalog say it runs a pair of 6V6 and made for a Deluxe Reverb, Deluxe Ampeg jet so I figure it's about 20 to 25 watts. I have one of these in an 18 watt, two EL84s and it's clean and loud.

I think I'll do what HBP suggested and rig the wiring without mounting the OT, first.

Hopefully I'll get it playing right, I'll post the results.

al

[HotBluePlates]

... I rewired back to 7591's,  and installed a TF110-48-UL from (Triode Elec/Magnetic.) that I had on-hand.  ... they have other 6.6K 20W OTs, and I've used several.  All of the amps they are installed in sound great,  including the Gem with the TF110-48-UL (did not use the UL taps,,).

The OT I have is a Magic Parts TF-110, it's 6.5K , 8 ohm tap. Looks about the same size as the original in the Reverbojet.

The specs in Ruby's catalog say it runs a pair of 6V6 and made for a Deluxe Reverb, Deluxe Ampeg jet so I figure it's about 20 to 25 watts. ...

Yep, this is why I recommended a Deluxe Reverb OT. Until recently, I didn't know they were ~6.6kΩ while the Princeton Reverb is 8kΩ. Since you mentioned the stock transformer was 6.5kΩ, I figured that to be a natural match.

[dude]

Yep, this is why I recommended a Deluxe Reverb OT. Until recently, I didn't know they were ~6.6kΩ while the Princeton Reverb is 8kΩ. Since you mentioned the stock transformer was 6.5kΩ, I figured that to be a natural match.

hey, how did you get the ohm insignia in your reply...?
I haven't had the time to try the new OT, will get to it later today.

thanks,
al

[HotBluePlates]

hey, how did you get the ohm insignia in your reply...?

On a windows machine, press and hold the ALT key. Key in 2 then 3 then 4. Release the ALT key. Ω will appear.

Or, you could pull up the Character Map (click Start button, enter "character map" in the search field, click on the Character Map program it returns). Select the symbol you want, click "Copy" then paste into the message you're typing for the forum. The character will appear.

[dude]

I'm changing out the OT in this Ampeg, as you can see from the schematic this amp has neg feedback, the original OT has two secondaries each from the OT, two black wires and two green wires, one set goes to the speaker and other is negative feedback.

My new OT only has one green 8 ohm and black sec to go the the speaker. To keep the negative feedback would I just splice into the the new OT's secondaries and run one set to the negative feedback and the other to the speaker? Then neg feedback is on all the time.

Also, I think I'll like to put in a switch to turn off the neg feedback, where would I put it and any changes to feedback components?

The N on the schematic off the neg feedback stands for neg feedback... correct?  Use the boards numbers on where to put the switch, I guess I could also put in a pot to dial in feedback, if I did that what value do I need? and where?
Thanks, al
 

[HotBluePlates]

... the original OT has two secondaries each from the OT, two black wires and two green wires, one set goes to the speaker and other is negative feedback.

My new OT only has one green 8 ohm and black sec to go the the speaker. ...

That's not what I see in the schematic. There is 1 secondary, meaning 1 winding between green and black. That said, there are 2 lines drawn from green and black, for a total of 4 lines, as you said.

My new OT only has one green 8 ohm and black sec to go the the speaker. To keep the negative feedback would I just splice into the the new OT's secondaries and run one set to the negative feedback and the other to the speaker?

No, do not "splice".

The correct way should be obvious from how the original transformer is mounted in the amp now. You should see the green and black wires (1 each) running to the speaker jack.

The jack also has the lug with the black wire grounded in some way, either by some connection to the chassis or to a ground on the circuit board. Any such connection is made by running a wire from the lug on the jack over to ground. In the same way, the green wire from the OT runs to the other lug on the speaker jack, then has an extra wire running from that lug back to the feedback components on the board (if the distance is short, the 270kΩ resistor beside the "N" may be the "wire" running between the jack and board.

You see, what I've described is electrically the same as splicing into the wires coming out of the transformer, but is a neater, safer and all-around better way of getting the job done.

You'll see the exact same method if you compare the layout to a schematic of a Fender amp with feedback.

[dude]

Yes, there is one sec, an 8 ohm tap but as you can see in the picture that one 8 ohm sec has two blk and two green wires, on top is the plates primary and B+.

Two wires go to a hanging speaker jack as you see, not grounded, the other two wires that look like they are connected all at the same winding go to the neg feedback, the black is grounded at the first power filter the other green goes to the 270K feedback loop, etc.

So you're saying because my new OT only has one secondaries set of wires, (8 ohms) that I should tap off the hanging jack's tip (green) to the 270K and take the black wire that was grounded at the filter, from the filter to the sleeve of the hanging jack?  I probably should mount the jack in the chassis for the ground and disconnect that black wire going to the first filter cap ground..?

I'm just a bit confused as I never encountered an OT with one sec winding and four wires..? tothe same winding.

al

[HotBluePlates]

Unsolder the black and green wires. Measure resistance from green to green, black to black. If it's less than 1Ω, wire up the new transformer as I recommended.

I've never seen a transformer like yours, when the single secondary has two wires per end. Very likely, where the winding ends underneath the insulation, two wires were soldered to the winding end instead of one.

...

So you're saying because my new OT only has one secondaries set of wires, (8 ohms) that I should ...

OT Green wire  ->  Speaker Jack +  ->  270kΩ resistor  ->  Negative feedback point in circuit

OT Black wire  ->  Speaker Jack -  ->  Ground (wherever is most convenient)

[dude]

Thanks, the two sets of the one winding I never saw either but I understand that splicing wires for a connection is a no/go.

Where would I put the switch to turn off the feedback, I assume right before the 270K resistor, a single thrown that just brakes the connection?

I appreciate your patience's,
al   

[HotBluePlates]

Where would I put the switch to turn off the feedback, I assume right before the 270K resistor, a single thrown that just brakes the connection?

The 270k resistor connects to an 8.2k resistor at V3 pin 8.

Your switch can be between the speaker jack + and the 270k resisotr, or between the 270k resistor and the ungrounded end of the 8.2k resistor. Either point does the same thing in breaking the connection from OT secondary to the point where feedback is injected in the circuit.

[dude]

I'm changing the OT's out right now and noticed someone had the + and - on the hanging speaker input jack reversed. I'm pretty sure with one speaker this doesn't make a difference but because this amp has negative feedback would the blk ground wire from the old OT going to the input of the feedback loop (the 270K R) and the green, + going to ground have anything to do with the OT sounding weak with low volume...?

al

Update

I put the new OT in as recommended by HBP with the Negative Feed Back on a switch, I mounted the speaker jack inside the amp on the chassis, that's the ground for the speaker (not hanging like the pic). I put the NFB switch off the speaker jack + and before the 270K R (both mounted next to each other in the chassis). This is exactly like the schematic except the old OT had one sec winding with two pairs of wires like the pic I posted. The new one only had a pair, + and -.

The resistance between both green and both black sec on the old OT were less than 1 ohm, so they both most likely were solder at the same winding point.

Feedback switch off, the amp screams, very loud and good head room, basically all fixed, looks like it was the old OT...?  But when I turn on the feedback, I get a "huge pulsating hum from the speaker", I can feel the vibration and have to turn it off in seconds.

There are only two components in the feedback loop, the 270K to the cathode of the PI or driver tube (12AX7) then from the cathode to ground through an 8.2K R. Both resistors check out good, I checked the solder joints and cleaned the board in that area.

Still the big buzz...?  Could switching the OT primaries, brown and blue possibly help? But the amp plays great with the feedback off.

al

[Glennjeff]

Swap

[dude]

Swap

Did the trick, fixed. Thanks, I guess I was getting sick of screwing around with this amp, getting new power tubes that were bad out of the box. I know a few times swapping the secondaries stopped squealing before in new amps I built but since the amp played with feedback off, I didn't think it could be that.

Anyway, these 7591's sound great with feedback on or off, amp is very loud. Those 7591 must put out around 25 watts at least.

The problem with the amp was the filters and the OT was weak, even though it checked out good. New OT, new filters, fixed. So, it looks like some of the old Ampeg OT's after a while get weak sounding even though they check out OK. Something good to know and I'm not the only one ,found a few posts on other forums with the same problem, weak volume on the OT.

Thanks, HBP for all your patience and help. I wish I could work in a repair shop for a month, free to learn more. It's tough reading on your own with no one to explain how circuits work but you don't forget that way...

al

[PRR]

> resistance between both green and both black sec on the old OT were less than 1 ohm

It always is.

Speaker resistance is an essential evil. Transformer winding resistance is BAD, except we can't afford infinitely conducty wire, so we tolerate "small" resistance. Much smaller than speaker resistance. For say 8 ohm load, we would like less than 0.8 ohm of winding resistance, and often get there. And of course most genral-purpose meters call that "zero".

That's DC resistance. The AC (audio) impedance is higher. If you must know, feed 6V _AC_ through about 10 ohms to the "8 ohm" winding and back. If you get several volts AC on the winding, it isn't shorted. If you get zero VAC on the winding it is hard-shorted. If you get less than a Volt there may be an indirect short (or the OT may just have poor bass at 60Hz).

[Glennjeff]

What is supposed to be negative feedback will be positive feedback if the secondaries are reversed wrt the feedback loop.

When they work OK without feedback but scream with feedback then reversed secondaries are first suspect.

Glad it's sounding fine now. Keep up the learning - This place is great for that.