Microphone In Guitar amp Question

Trout · 28th June 2006, 01:21 PM

I have a really nice little 5E3 clone thats a gas to play thru, But, I am in need of a nice harp amp. These were pretty legendary with a lot of old blues players.

The Problem is, no matter how large the room is, If I raise the volume over 1/10th I get terrible feedback. Even at over 15ft away.

Is there a simple mod to corect this type of thing?
5E3 schematic

I have no experience with this type of setup, But it seems to me it shouldnt be to hard to correct if I knew where to look.
Trout

bibster · 28th June 2006, 01:41 PM

found this quote:

Quote:

Tube swapping modifications easily done on the Bassman RI further optimize it for use in amplified harp play--specifically the solid state rectifier is often replaced with a tube rectifier, and the stock 12AX7 tubes are replaced with lower-gain substitutes such as the 12AT7 or even the low gain 12AU7. These tube substitutions allow the amp's volume knob to be turned to higher settings before feedback occurs, which provides more "headroom" for volume adjustments that don't result in feedback. They also contribute to an earlier (less volume) breakup into the sought after tube overdrive distortion.

over here...
(You're not using a bassman, but hey..)

You could try that one quite easily, not?

Cheers, Paul

Giaime · 28th June 2006, 01:42 PM

Hello Trout!

I've read somewhere an article about guitar -> harp conversion in tube amps, but I don't remember, maybe I'll post it later if I find it.

However, I would try adding some negative feedback, in general that always helps against acoustic feedback.

Also, try to limit frequency response of the amp: with the harp you don't need such low notes and high ones. So if with guitar you need about 50Hz - 10kHz, I would cut anything lower than 100Hz and anything higher than 5kHz.

Try some negative feedback from the output transformer secondary to the cathode of the first section of the 12AX7 in the schematic. Use a resistor (a pot) to set a variable resistive divider in conjunction with the 1500ohm cathode resistor of that stage.

Hint: if the amp starts to oscillate this way, without input, swap output tubes anode connections to the OTP.

Enzo · 29th June 2006, 02:40 AM

And keep in mind that it is not the amp that feeds back, it is the system. The ultimate cause of feedback is the mic picking up what comes out the speaker and sending it back through the system.

So a lot can be done simply by working with the amp placement as well as with proper mic technique.

Obviously lowering the gain of the amp will make it less likely to feed back, but the conditions for feedback have to be in place for it to occur.

Trout · 29th June 2006, 02:13 PM

Thanks for all the great replies,

I think possibly I have located a possibility as to why at least part of the feedback may be occuring.

When I built the amp, I only installed 2 inputs instead of 4.
Making them basically a bright and normal setup.

Looking back at the original 5E3 layout and schematic I omitted the lower set of microphone inputs.

They should be wired like this.

The lower 2 jacks are actually wired different than the upper pair.

The jacks I used are Cliff Jacks ( Marshall style) that are switched (normally closed). So it looks to me like part of the problem is the 1 meg resistor?

When a plug is inserted into the lower jack, it opens the switch and has no 1 meg resistor from tip to ground, or at thats what I think I am seeing?
I have a second amp here with all 4 jacks, The lower pair both have less output than the upper pair.
I am however still needing to run 12AT7's instead of the 12AX7's. The Normal channel is working better than the bright channel which is to be expected.

Trout

poobah · 29th June 2006, 04:47 PM

That could indeed be your problem Gene. The mic might want to look into a lower inpedance. What type of mic are you using?

Nigel Goodwin · 29th June 2006, 04:55 PM

Miking a harp for PA sounds a truely horrendous task!, the harp itself isn't really very loud, but it is BIG - so you can't get the mike too close, or the closest strings will be louder than the further ones.

If you have the mike further away, the gain needs turning up further, and feedback is almost unavoidable!.

Did PA last Christmas for my daughter (bass) and Charlie (guitar), plus two vocal mikes - but both bass and guitar were acoustic with in-built mikes. It was SO difficult to try and get enough volume without feedback, so we had a LOT of feedback throughout the night!. I told them afterwards, "we're NEVER doing that again!" - did another 'acoustic' duo gig a couple of weeks back - didn't take acoustic guitars, used electric ones instead!

Trout · 29th June 2006, 05:09 PM

Quote:

Originally posted by poobah
That could indeed be your problem Gene. The mic might want to look into a lower inpedance. What type of mic are you using?

ASTATIC JT-30 Ceramic, High Impendance Real old school lol
trout

YEP, Harmonica

poobah · 29th June 2006, 05:10 PM

Good point... are you talking about a harp or a harmonica?

Build a small adaptor with a 1/4" phone plug & jack. Try anywhere from 10K to 100K from tip to ring.

The high impedance might be driving the input tube nuts.

Enzo · 30th June 2006, 02:51 AM

Your 1 meg resistor winds up in parallel with the mic impedance. I doubt it makes much difference.

28th June 2006, 01:21 PM	#1
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	Microphone In Guitar amp Question I have a really nice little 5E3 clone thats a gas to play thru, But, I am in need of a nice harp amp. These were pretty legendary with a lot of old blues players. The Problem is, no matter how large the room is, If I raise the volume over 1/10th I get terrible feedback. Even at over 15ft away. Is there a simple mod to corect this type of thing? 5E3 schematic I have no experience with this type of setup, But it seems to me it shouldnt be to hard to correct if I knew where to look. Trout

29th June 2006, 04:55 PM	#7
Nigel Goodwin diyAudio Member Join Date: Jul 2005 Location: North Derbyshire	Miking a harp for PA sounds a truely horrendous task!, the harp itself isn't really very loud, but it is BIG - so you can't get the mike too close, or the closest strings will be louder than the further ones. If you have the mike further away, the gain needs turning up further, and feedback is almost unavoidable!. Did PA last Christmas for my daughter (bass) and Charlie (guitar), plus two vocal mikes - but both bass and guitar were acoustic with in-built mikes. It was SO difficult to try and get enough volume without feedback, so we had a LOT of feedback throughout the night!. I told them afterwards, "we're NEVER doing that again!" - did another 'acoustic' duo gig a couple of weeks back - didn't take acoustic guitars, used electric ones instead! __________________ Nigel Goodwin

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28th June 2006, 01:42 PM	#3
Giaime diyAudio Member Join Date: Oct 2004	Hello Trout! I've read somewhere an article about guitar -> harp conversion in tube amps, but I don't remember, maybe I'll post it later if I find it. However, I would try adding some negative feedback, in general that always helps against acoustic feedback. Also, try to limit frequency response of the amp: with the harp you don't need such low notes and high ones. So if with guitar you need about 50Hz - 10kHz, I would cut anything lower than 100Hz and anything higher than 5kHz. Try some negative feedback from the output transformer secondary to the cathode of the first section of the 12AX7 in the schematic. Use a resistor (a pot) to set a variable resistive divider in conjunction with the 1500ohm cathode resistor of that stage. Hint: if the amp starts to oscillate this way, without input, swap output tubes anode connections to the OTP.

29th June 2006, 02:40 AM	#4
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	And keep in mind that it is not the amp that feeds back, it is the system. The ultimate cause of feedback is the mic picking up what comes out the speaker and sending it back through the system. So a lot can be done simply by working with the amp placement as well as with proper mic technique. Obviously lowering the gain of the amp will make it less likely to feed back, but the conditions for feedback have to be in place for it to occur.

29th June 2006, 02:13 PM	#5
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	Thanks for all the great replies, I think possibly I have located a possibility as to why at least part of the feedback may be occuring. When I built the amp, I only installed 2 inputs instead of 4. Making them basically a bright and normal setup. Looking back at the original 5E3 layout and schematic I omitted the lower set of microphone inputs. They should be wired like this. The lower 2 jacks are actually wired different than the upper pair. The jacks I used are Cliff Jacks ( Marshall style) that are switched (normally closed). So it looks to me like part of the problem is the 1 meg resistor? When a plug is inserted into the lower jack, it opens the switch and has no 1 meg resistor from tip to ground, or at thats what I think I am seeing? I have a second amp here with all 4 jacks, The lower pair both have less output than the upper pair. I am however still needing to run 12AT7's instead of the 12AX7's. The Normal channel is working better than the bright channel which is to be expected. Trout

29th June 2006, 04:47 PM	#6
poobah diyAudio Member Join Date: Nov 2005	That could indeed be your problem Gene. The mic might want to look into a lower inpedance. What type of mic are you using?

29th June 2006, 05:10 PM	#9
poobah diyAudio Member Join Date: Nov 2005	Good point... are you talking about a harp or a harmonica? Build a small adaptor with a 1/4" phone plug & jack. Try anywhere from 10K to 100K from tip to ring. The high impedance might be driving the input tube nuts.

30th June 2006, 02:51 AM	#10
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Your 1 meg resistor winds up in parallel with the mic impedance. I doubt it makes much difference.

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