Strange Amp problem

bigpony777

bigpony777

New member
Hi, i recently got a super cheap crate td70- i was super excited about it because it has real spring reverb and tubes! sounded like an upgrade from my old stuff- i got it and the thing is broken. DI works perfectly- but both channels have a loud hum and output at half volume. the only tube i can see lights up and looks fine- this is making me think there is a problem with some connections being dirty and rusted. if the tube was jacked the second channel would still be fine right? i figured they were totally independent and channel B wouldnt run through the tube, but i could be mistaken. Any thoughts on this would be appreciated- I may buy a tube to test it but fear that will be a waste of 25 bucks.
 
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AFAICT the DT 70 is a hybrid guitar amplifier that has a single pre amp valve (almost certainly an ECC83/12AX7) that drives a solid state power amp stage. Such designs are not easily serviced by the average Joe!

The loud hum could be a filter cap' but in my exp' more likely a fault in the power amp stage and if you run it for more than a few minutes it will likely fail beyond economic repair. Transistor power stages can be b'stds to fix and if it were brought to me I would suggest scrapping the power stage and fitting a modern PA module.

Some photos of the internals could verify my suspicions. I shall also ask on a British guitar amp forum. A few people there know EVERYTHING about EVERY amp EVER made!

Dave.
 
bigpony777 said:
Hi, i recently got a super cheap crate dt70- i was super excited about it because it has real spring reverb and tubes! sounded like an upgrade from my old stuff- i got it and the thing is broken. DI works perfectly- but both channels have a loud hum and output at half volume. the only tube i can see lights up and looks fine- this is making me think there is a problem with some connections being dirty and rusted. if the tube was jacked the second channel would still be fine right? i figured they were totally independent and channel B wouldnt run through the tube, but i could be mistaken. Any thoughts on this would be appreciated- I may buy a tube to test it but fear that will be a waste of 25 bucks.
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Pretty sure the 12ax7 would cover both channels, as it is advertised as having a tube preamp. You should be able to get one at a local shop for 15-20 dollars. Worth a shot. As mentioned , some problems will cost a few just to have a tech find them, and often can be more expensive than the amps original cost. Changing out the preamp tube is something you can do yourself for very little costs, and could solve the issue
 
SHEPPARDB. said:
Pretty sure the 12ax7 would cover both channels, as it is advertised as having a tube preamp. You should be able to get one at a local shop for 15-20 dollars. Worth a shot. As mentioned , some problems will cost a few just to have a tech find them, and often can be more expensive than the amps original cost. Changing out the preamp tube is something you can do yourself for very little costs, and could solve the issue
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Fair enough but if simply removing the valve does not stop the hum it is not the problem. Preamp valves tend to last decades.

Dave.
 
ecc83 said:
AFAICT the DT 70 is a hybrid guitar amplifier that has a single pre amp valve (almost certainly an ECC83/12AX7) that drives a solid state power amp stage. Such designs are not easily serviced by the average Joe!

The loud hum could be a filter cap' but in my exp' more likely a fault in the power amp stage and if you run it for more than a few minutes it will likely fail beyond economic repair. Transistor power stages can be b'stds to fix and if it were brought to me I would suggest scrapping the power stage and fitting a modern PA module.

Some photos of the internals could verify my suspicions. I shall also ask on a British guitar amp forum. A few people there know EVERYTHING about EVERY amp EVER made!

Dave.
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i actually know its not the power amp because the DI is still working as I said- that just goes directly into the power amp.
 
ecc83 said:
Sorry, that last sentence confuses me?

Dave.
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my bad haha i do not know the proper terminology- basically- there is a direct input on this amp that skips the preamp and all effects it bypasses them straight to the power amp. this works fine so i know the power amp isnt broken- although a connection to it from the preamp could be broken.
 
bigpony777 said:
my bad haha i do not know the proper terminology- basically- there is a direct input on this amp that skips the preamp and all effects it bypasses them straight to the power amp. this works fine so i know the power amp isnt broken- although a connection to it from the preamp could be broken.
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Still groping! Many of the older amps had a "pre out" and a "PA in"pair of jacks that were 'normalized. If the link was broken and the PA humming, it would still hum. Also, is the hum affected by the volume pot?

Dave.
 
Here is the schematic for anyone interested.
 

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dogooder said:
Here is the schematic for anyone interested.
Click to expand...
ecc83 said:
Still groping! Many of the older amps had a "pre out" and a "PA in"pair of jacks that were 'normalized. If the link was broken and the PA humming, it would still hum. Also, is the hum affected by the volume pot?

Dave.
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thanks for that schematic, very helpful! no the hum is not affected by drive or level knobs.
 
If you plug a guitar into "PA in" does the hum stop? If so the PA is in fact OK assuming you get some sound. Won't be much as there is only about 25dB of gain there.
Very suspect is the 10W zener feed resistors and the zeners. 'Twere mine I would replace the Jewson lot with 'modern' 78 series regulators. (I would also 'DC heat' the valve but that's just one of my "things"!)

Oh yes! Sorry! VERY good work getting us that map.

Dave
 
ecc83 said:
If you plug a guitar into "PA in" does the hum stop? If so the PA is in fact OK assuming you get some sound. Won't be much as there is only about 25dB of gain there.
Very suspect is the 10W zener feed resistors and the zeners. 'Twere mine I would replace the Jewson lot with 'modern' 78 series regulators. (I would also 'DC heat' the valve but that's just one of my "things"!)

Oh yes! Sorry! VERY good work getting us that map.

Dave
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hmmm by PA do you mean into the DI? if so then yes the DI is totally clean and loud sounds perfect.
 
"hmmm by PA do you mean into the DI?" I think so but we may be at cross purposes? "DI" is usually reserved for the High Z input on an interface or sometimes a mixer. Then there is the "DI box", not a term usually found on amplifiers except maybe as "DI out" often but wrongly IMHO, on an XLR.

So, yes, could be the valve as the stupid *****ds have fed the heater from AC when there is a perfectly good DC LV supply they could have used! What happens if you pull the valve?

I am sorry for all the 'pedantry' but remote diagnoses is almost impossible enough! We all need to sing from the same song book!

Dave.
 
ecc83 said:
"hmmm by PA do you mean into the DI?" I think so but we may be at cross purposes? "DI" is usually reserved for the High Z input on an interface or sometimes a mixer. Then there is the "DI box", not a term usually found on amplifiers except maybe as "DI out" often but wrongly IMHO, on an XLR.

So, yes, could be the valve as the stupid *****ds have fed the heater from AC when there is a perfectly good DC LV supply they could have used! What happens if you pull the valve?

I am sorry for all the 'pedantry' but remote diagnoses is almost impossible enough! We all need to sing from the same song book!

Dave.
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here is the manual- these are the terms im going off of- by DI i mean the direct input to the power amp. sorry its taking me so long to respond im in class ATM but i will test the removed valve when im back in my dorm. https://www.manualslib.com/manual/552278/Crate-Td70.html?page=3#manual
 
ecc83 said:
"hmmm by PA do you mean into the DI?" I think so but we may be at cross purposes? "DI" is usually reserved for the High Z input on an interface or sometimes a mixer. Then there is the "DI box", not a term usually found on amplifiers except maybe as "DI out" often but wrongly IMHO, on an XLR.

So, yes, could be the valve as the stupid *****ds have fed the heater from AC when there is a perfectly good DC LV supply they could have used! What happens if you pull the valve?

I am sorry for all the 'pedantry' but remote diagnoses is almost impossible enough! We all need to sing from the same song book!

Dave.
Click to expand...
oh wow- i realized ive been saying DI instead of line in this whole time my bad!! I just removed the tube and the hum is still there volume is still the same.
 
SHEPPARDB. said:
The old ones do, but much of the newer preamp tubes are microphonic right out of the box, if they function at all.By old, I don’t mean in the era these amps were built in.??
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Well they must have gone downhill quite a piece since I was using them ten years ago? We used to buy 50 ECC83s in at a time and 45 of them were perfectly fine in all positions but I used to select the ones with lowest noise and hum for front ends. Had no failures as I recall. Sovteks, TAD and other makes.

Dave.
 
The two inputs on the front are together, the only difference being one is slightly less sensitive. Both go through the same preamp path.

There's a "line out" to give the ability to go direct to a mixer or tape, and a "line in" to be used with the out for an effects loop. From the schematic, plugging into the Line In jack will disconnect the preamp section from the circuit. (upper left of the schematic.). Plugging into the Line Out still preserves the signal path to the amp.

It appears that you could test the preamp section by running it to another amp or mixing board. If that hums, then you know the issue is on the bottom page of the schematic. If you pull the tube and the hum is still there, then you are probably looking between test point 3 and test point 9. I would take some time to check those test points to see where things go wonky.
 
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