1966 JTM-45 Problem

1966 JTM-45 Problem

Hi,

A guy brought me his 1966 JTM45.
He tried to service it himself and wired the filter caps wrong.
He also installed a bias pot (the right way).

The thing is, I get no bias control at all, no sound at all and a very
loud motorboating when all controls are all the way down. When I dial
the controls about half-way up, the motorboating stops.

I can't figure out what's wrong with this amp.

Any leads appreciated.

Steve

Re: 1966 JTM-45 Problem


"Steve" <stephen@valveart-tech . com > wrote in message
news:482ad7da$0$13028$426a74cc@news . fr ee . fr ...
> Hi,
>
> A guy brought me his 1966 JTM45.
> He tried to service it himself and wired the filter caps wrong.
> He also installed a bias pot (the right way).
>
> The thing is, I get no bias control at all, no sound at all and a very
> loud motorboating when all controls are all the way down. When I dial the
> controls about half-way up, the motorboating stops.
>
> I can't figure out what's wrong with this amp.
>
> Any leads appreciated.
>
> Steve
>

Motor boating is typically a sign of a bad filter cap.You say he wired them
wrong. Is that just connected to the wrong spot or did he flip the pos and
the neg terminals? Did you correct it and reuse them? Try new ones. No
saying what collateral damage might be, as info provided lacks details.

No sound could be lots of things, but essentitally the circuit is open
somewhere and you've got to find where you get zero instead of expected
voltage. Start by probing tube pins?

You say he did the bias mod right, but did he fry the pot (or something
else) with too much heat? Once again, probe for voltage (power tubes
pulled, pot maxed) and see where you lose it. Is there any bias voltage
present and you just can't control it with the pot or is there no bias
voltage?

Can you post some pics somewhere?

Re: 1966 JTM-45 Problem

On 2008-05-14 15:16:59 +0200, "Phil S." <psymonds no spam@comcast . net > said:

>
> "Steve" <stephen@valveart-tech . com > wrote in message
> news:482ad7da$0$13028$426a74cc@news . fr ee . fr ...
>> Hi,
>>
>> A guy brought me his 1966 JTM45.
>> He tried to service it himself and wired the filter caps wrong.
>> He also installed a bias pot (the right way).
>>
>> The thing is, I get no bias control at all, no sound at all and a very
>> loud motorboating when all controls are all the way down. When I dial the
>> controls about half-way up, the motorboating stops.
>>
>> I can't figure out what's wrong with this amp.
>>
>> Any leads appreciated.
>>
>> Steve
>>
>
> Motor boating is typically a sign of a bad filter cap.You say he wired them
> wrong. Is that just connected to the wrong spot or did he flip the pos and
> the neg terminals? Did you correct it and reuse them? Try new ones. No
> saying what collateral damage might be, as info provided lacks details.
>
> No sound could be lots of things, but essentitally the circuit is open
> somewhere and you've got to find where you get zero instead of expected
> voltage. Start by probing tube pins?
>
> You say he did the bias mod right, but did he fry the pot (or something
> else) with too much heat? Once again, probe for voltage (power tubes
> pulled, pot maxed) and see where you lose it. Is there any bias voltage
> present and you just can't control it with the pot or is there no bias
> voltage?
>
> Can you post some pics somewhere?

Hi Phil,

Yes, I know motorboating is a tipical sign of bad filter caps.
He did not flip the neg and pos sides. I did not reuse them. I
changed them altogether.
I changed the bias pot as well as the entire bias circuit. There is
bias current (very high 75mA) and Plate voltage around 500VDC. But the
pot has no effect at all. My scope is dead so I'm having a hard time
tracing down the problem.

I'll post some pics later on today.

Thanks for your help,

Steve
--
Make the world a better place.
Stop being an asshole

Re: 1966 JTM-45 Problem


"Steve" <stephen@valveart-tech . com > wrote in message
news:482af3a1$0$2027$426a74cc@news . fr ee . fr ...
> On 2008-05-14 15:16:59 +0200, "Phil S." <psymonds_no_spam@comcast . net >
> said:
>
>>
>> "Steve" <stephen@valveart-tech . com > wrote in message
>> news:482ad7da$0$13028$426a74cc@news . fr ee . fr ...
>>> Hi,
>>>
>>> A guy brought me his 1966 JTM45.
>>> He tried to service it himself and wired the filter caps wrong.
>>> He also installed a bias pot (the right way).
>>>
>>> The thing is, I get no bias control at all, no sound at all and a very
>>> loud motorboating when all controls are all the way down. When I dial
>>> the
>>> controls about half-way up, the motorboating stops.
>>>
>>> I can't figure out what's wrong with this amp.
>>>
>>> Any leads appreciated.
>>>
>>> Steve
>>>
>>
>> Motor boating is typically a sign of a bad filter cap.You say he wired
>> them
>> wrong. Is that just connected to the wrong spot or did he flip the pos
>> and
>> the neg terminals? Did you correct it and reuse them? Try new ones. No
>> saying what collateral damage might be, as info provided lacks details.
>>
>> No sound could be lots of things, but essentitally the circuit is open
>> somewhere and you've got to find where you get zero instead of expected
>> voltage. Start by probing tube pins?
>>
>> You say he did the bias mod right, but did he fry the pot (or something
>> else) with too much heat? Once again, probe for voltage (power tubes
>> pulled, pot maxed) and see where you lose it. Is there any bias voltage
>> present and you just can't control it with the pot or is there no bias
>> voltage?
>>
>> Can you post some pics somewhere?
>
> Hi Phil,
>
> Yes, I know motorboating is a tipical sign of bad filter caps.
> He did not flip the neg and pos sides. I did not reuse them. I changed
> them altogether.
> I changed the bias pot as well as the entire bias circuit. There is bias
> current (very high 75mA) and Plate voltage around 500VDC. But the pot has
> no effect at all. My scope is dead so I'm having a hard time tracing down
> the problem.
>
> I'll post some pics later on today.
>
> Thanks for your help,
>
> Steve

Steve,
I'm an amateur, but I did build a JTM45 and it's a heck of a nice amp if
mine's any indication. You probably know more than me if you repair things
for a living. I didn't mean to overstate the obvious, but you didn't give
much to go on.

The bias circuit troubles me. I think you know, 75mA is way too much,
meaning that it is not (or no longer) properly constructed. I'd guess grid
current would be very low, maybe 5mA? I don't know as I never thought to
measure it on any amp I've got (3 fixed bias + 6 cathode bias; 7 of them are
homebrew)..

What votage are you seeing at these points: between the HT secondary and the
180K (or 220K) dropping resistor (AC); after the 180K (220K) dropping
resistor (AC); after the diode (DC). After that there would be a 15K --
what's the DCV there. at the junction with the 68K? Exactly what's there
now and how is it configured (the bias pot)? What I'm getting at is that
even though you think it is done correctly, it may not be. Chances are you
aren't seeing that it is grounded somewhere it shouldn't be or otherwise
boluxed..

You don't say...is there any negative grid voltage from the bias circuit?

I sent you some email that might help.

Re: 1966 JTM-45 Problem

On 2008-05-14 23:22:55 +0200, "Phil S." <psymonds no spam@comcast . net > said:

>
> "Steve" <stephen@valveart-tech . com > wrote in message
> news:482af3a1$0$2027$426a74cc@news . fr ee . fr ...
>> On 2008-05-14 15:16:59 +0200, "Phil S." <psymonds no spam@comcast . net >
>> said:
>>
>>>
>>> "Steve" <stephen@valveart-tech . com > wrote in message
>>> news:482ad7da$0$13028$426a74cc@news . fr ee . fr ...
>>>> Hi,
>>>>
>>>> A guy brought me his 1966 JTM45.
>>>> He tried to service it himself and wired the filter caps wrong.
>>>> He also installed a bias pot (the right way).
>>>>
>>>> The thing is, I get no bias control at all, no sound at all and a very
>>>> loud motorboating when all controls are all the way down. When I dial
>>>> the
>>>> controls about half-way up, the motorboating stops.
>>>>
>>>> I can't figure out what's wrong with this amp.
>>>>
>>>> Any leads appreciated.
>>>>
>>>> Steve
>>>>
>>>
>>> Motor boating is typically a sign of a bad filter cap.You say he wired
>>> them
>>> wrong. Is that just connected to the wrong spot or did he flip the pos
>>> and
>>> the neg terminals? Did you correct it and reuse them? Try new ones. No
>>> saying what collateral damage might be, as info provided lacks details.
>>>
>>> No sound could be lots of things, but essentitally the circuit is open
>>> somewhere and you've got to find where you get zero instead of expected
>>> voltage. Start by probing tube pins?
>>>
>>> You say he did the bias mod right, but did he fry the pot (or something
>>> else) with too much heat? Once again, probe for voltage (power tubes
>>> pulled, pot maxed) and see where you lose it. Is there any bias voltage
>>> present and you just can't control it with the pot or is there no bias
>>> voltage?
>>>
>>> Can you post some pics somewhere?
>>
>> Hi Phil,
>>
>> Yes, I know motorboating is a tipical sign of bad filter caps.
>> He did not flip the neg and pos sides. I did not reuse them. I changed
>> them altogether.
>> I changed the bias pot as well as the entire bias circuit. There is bias
>> current (very high 75mA) and Plate voltage around 500VDC. But the pot has
>> no effect at all. My scope is dead so I'm having a hard time tracing down
>> the problem.
>>
>> I'll post some pics later on today.
>>
>> Thanks for your help,
>>
>> Steve
>
> Steve,
> I'm an amateur, but I did build a JTM45 and it's a heck of a nice amp if
> mine's any indication. You probably know more than me if you repair things
> for a living. I didn't mean to overstate the obvious, but you didn't give
> much to go on.
>
> The bias circuit troubles me. I think you know, 75mA is way too much,
> meaning that it is not (or no longer) properly constructed. I'd guess grid
> current would be very low, maybe 5mA? I don't know as I never thought to
> measure it on any amp I've got (3 fixed bias + 6 cathode bias; 7 of them are
> homebrew)..
>
> What votage are you seeing at these points: between the HT secondary and the
> 180K (or 220K) dropping resistor (AC); after the 180K (220K) dropping
> resistor (AC); after the diode (DC). After that there would be a 15K --
> what's the DCV there. at the junction with the 68K? Exactly what's there
> now and how is it configured (the bias pot)? What I'm getting at is that
> even though you think it is done correctly, it may not be. Chances are you
> aren't seeing that it is grounded somewhere it shouldn't be or otherwise
> boluxed..
>
> You don't say...is there any negative grid voltage from the bias circuit?
>
> I sent you some email that might help.

Phil,

Thanks for the help.
You're right, there might be something wrong with bias circuit I am not seeing.
Obviously there is something wrong I'm not seeing, and it is frustrating. ;-)
I'll try to post some pics and the voltages later on. Very busy at the moment.

Thanks for the email, ;-)

Cheers,

Steve
--
Make the world a better place.
Stop being an asshole

Re: 1966 JTM-45 Problem

I know this might be a wacky idea but - - - could the motorboating be
a result of a bad ground connection somewhere? Even if the B+ cap
string is all properly hooked up, a bad ground somewhere could
possibly cause similar behavior. And if the guy was wiring his own
caps in and did it wrong, he might have connected a ground terminal to
the wrong thing. Just another random thing to try.

Re: 1966 JTM-45 Problem


<morris.slutsky@gmail . com > wrote in message
news:55712260-6c40-4153-a803-9f623284c30d@j22g2000hsf.googlegroups . com ...
>I know this might be a wacky idea but - - - could the motorboating be
> a result of a bad ground connection somewhere? Even if the B+ cap
> string is all properly hooked up, a bad ground somewhere could
> possibly cause similar behavior. And if the guy was wiring his own
> caps in and did it wrong, he might have connected a ground terminal to
> the wrong thing. Just another random thing to try.

You're right. It could also be loose ground connection
from the filter caps but it also has a problem with the
bias circuit. Whenever an amp comes in for me to work
that some DIY guy has mucked around in, the first thing
that I deal with when it's time to work on it, is the bleedin'
SCHEMATIC.

Re: 1966 JTM-45 Problem


"Elvis Kabong" <ampscience@tuneland . com > wrote in message
news:g9KWj.5398$hv2.2877@bignews5.bellsouth . net ...
>
> <morris.slutsky@gmail . com > wrote in message
> news:55712260-6c40-4153-a803-9f623284c30d@j22g2000hsf.googlegroups . com ...
>>I know this might be a wacky idea but - - - could the motorboating be
>> a result of a bad ground connection somewhere? Even if the B+ cap
>> string is all properly hooked up, a bad ground somewhere could
>> possibly cause similar behavior. And if the guy was wiring his own
>> caps in and did it wrong, he might have connected a ground terminal to
>> the wrong thing. Just another random thing to try.
>
> You're right. It could also be loose ground connection
> from the filter caps but it also has a problem with the
> bias circuit. Whenever an amp comes in for me to work
> that some DIY guy has mucked around in, the first thing
> that I deal with when it's time to work on it, is the bleedin'
> SCHEMATIC.
>
>
I sent him what I think is the correct schematic. I had collected a few
when I built mine. I sent the one with the bias pot included. It is a
classic Marshall bias circuit. I'm ready to bet a dollar to a donut hole
there is either a munged ground or that bias pot is wired *all wrong*.

Look on the bright side. I hope the tech (Steve) has a proper rate sign
posted: Labor rate: $50/hour. $100/hour you wanna watch. $150/hour you
fixed it first.

Re: 1966 JTM-45 Problem

Ignore all other symptoms, and work on correcting the bias. Until the power
suplly is working correctly, the tone can't be right. Pretend you are
building the amp from scratch, and completely rebuild the bias if need be to
get it working correctly. Then see which of the other symptoms remain - many
tonal problems might go away once the bias has been corrected. And don't
forget to charge an arm and a leg for undoing this DIY guy's mistakes. Its
not punishment (we all go through a learning curve), but it is only fair to
you.


"Steve" <stephen@valveart-tech . com > wrote in message
news:482ad7da$0$13028$426a74cc@news . fr ee . fr ...
> Hi,
>
> A guy brought me his 1966 JTM45.
> He tried to service it himself and wired the filter caps wrong.
> He also installed a bias pot (the right way).
>
> The thing is, I get no bias control at all, no sound at all and a very
> loud motorboating when all controls are all the way down. When I dial the
> controls about half-way up, the motorboating stops.
>
> I can't figure out what's wrong with this amp.
>
> Any leads appreciated.
>
> Steve
>