My new toy - Ampeg B-18-N head

[beans-on-toast]

Very nice amp. Your description of the new wiring of the NFB and speaker out appears to be correct.

Perhaps you have an impedance mismatch between the B-18 and the Carvin power amp. The intent of the ext amp connection is to run a cable to the input of another amp or to connect to the ext amp jack on another amp. I've run my early model B15NA into the input of different power amps without any problems. My circuit is not the same as yours though.

You could try pulling the 7199 phase inverter tube, connect your pre-amp to the Carvin power amp input and see what happens. Some of the load would be relieved. Try measuring the output signal level of the B-18 with a scope. It should be at least 1V RMS to fully drive the power amp. An impedance mismatch would reduce the signal level.

First check that you have an instrument cable, not a speaker cable running between the two amps to ensure that excessive noise will not be picked up. Just to be safe, test the Carvin with a line out level input signal other than the B-18 but use the same cable that you would use with the B18. Check that the input to the Carvin is set up to accept the type of signal that you are feeding it from the B18. I am not familiar with you power amp, but sometimes there are input options. Check that it is not padded (sometimes this is an internal jumper) or not expecting a balanced input.

Hope that this helps.

 

[aborgman]

Very nice amp. Your description of the new wiring of the NFB and speaker out appears to be correct.

Yeah, I'm pretty sure that part is fine - or at least it matches the schematic.

Perhaps you have an impedance mismatch between the B-18 and the Carvin power amp. The intent of the ext amp connection is to run a cable to the input of another amp or to connect to the ext amp jack on another amp. I've run my early model B15NA into the input of different power amps without any problems. My circuit is not the same as yours though.

Bingo.

Too low of an input impedance.

I assumed the ext. amp output would be able to drive a 20k load - and it appears I was wrong.

The source impedance is too high - likely because of the very circuit difference you mention.

The B-15-N is:

gain stage -> tone stack -> gain stage -> volume pot-> ext amp

The B-18-N is:

gain stage -> volume pot -> gain stage -> tone stack -> ext amp


Tone stacks generally have rather high output impedances.

I'm sure it will drive a tube input fine (input impedance in the mega-ohm range), but it isn't going to drive a 20k input impedance.

I drew up both circuits in spice - and adding a 20k load to the ext. amp node causes a signal loss of ~26dB.

You could try pulling the 7199 phase inverter tube, connect your pre-amp to the Carvin power amp input and see what happens. Some of the load would be relieved.

Not really - the input to the control grid of the pentode section of the 7199 is going to have an impedance in the mega-ohms range. Removing it will have zero effect when the 20k load is also connected.

First check that you have an instrument cable, not a speaker cable running between the two amps to ensure that excessive noise will not be picked up. Just to be safe, test the Carvin with a line out level input signal other than the B-18 but use the same cable that you would use with the B18. Check that the input to the Carvin is set up to accept the type of signal that you are feeding it from the B18. I am not familiar with you power amp, but sometimes there are input options. Check that it is not padded (sometimes this is an internal jumper) or not expecting a balanced input.

Hope that this helps.

All good ideas, and all things I had already checked.

I should have just cranked up Spice and sim'ed it earlier.

 

[JimmyM]

just spitballing here, no idea if it works, but could you plug into your power amp by jumping a channel from the b-18n?

 

[beans-on-toast]

Not really - the input to the control grid of the pentode section of the 7199 is going to have an impedance in the mega-ohms range. Removing it will have zero effect when the 20k load is also connected.

I should have just cranked up Spice and sim'ed it earlier.

You gotta love SPICE.

Right about the tube input impedance being mega high. I thought, just in case something really strange is going on within your particular 7199 or the socket or wiring around it. Stranger things have happened and it is an easy thing to eliminate when troubleshooting. The possibility is remote though.