Help Ampeg B-15NC B+ voltages low when pluging 6L6 Power Tubes

[Dom65]

Just an update to say that the amp is, finally, in great shape.
The intermittent sound cut was due to a bad bypass capacitor on the cathode of the PI.

The owner would like to be able to use the ext. amp output to record without plugging a speaker. Would one of a 50 w 8 ohm like these

https://www.amazon.ca/ETOPARS-Aluminium-Resistors-Wirewound-Heatshink/dp/B0BWTM171J/ref=asc_df_B0BWTM171J/?tag=googleshopc0c-20&linkCode=df0&hvadid=706827341474&hvpos=&hvnetw=g&hvrand=570941007532149989&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9000434&hvtargid=pla-2297319995429&mcid=fe9df6b1f2e13e138280bb6ba3a4ffad&gad_source=1&th=1

do the job?
I was thinking in fixing it in a guitar pedal casing with a 1/4 inch plug. Would that be safe to operate the amp with the speaker out plugged to such a box?

 

[agedhorse]

That might get too hot to safely touch… and maybe melt nearby objects like carpet.

It depends on how much power and for how long. At the temperature of the enclosure increases, the power handling of the resistor decreases.

 

[beans-on-toast]

Electronic parts from amazon are like a box of chocolates. You never know what you are getting. They sell a lot of fake sub-standard parts. Mouser.com (or Digikey or Newark) is better.

 

[beans-on-toast]

For a 25W NC@8 ohms, when turned up, expect around 1.77 A RMS, 2.5 A peak, 14.14 V RMS, 40 V peak-to-peak. The data sheet for the resistor will provide information on heat dissipation over a specified time. It should have a heat sink which can fit in a case. For amp safety reasons, locking plug would be better than a simple ¼” plug.

 

[agedhorse]

Electronic parts from amazon are like a box of chocolates. You never know what you are getting. They sell a lot of fake sub-standard parts. Mouser.com (or Digikey or Newark) is better.

Especially with the special “heatSHINK” construction.

 

[Dom65]

That might get too hot to safely touch… and maybe melt nearby objects like carpet.

The pedal casing in which it will be fixed should help dissipate heat. I guess I'm gonna have to test it.

It depends on how much power and for how long. At the temperature of the enclosure increases, the power handling of the resistor decreases.

Well that would be for the B15NC which is rated at 30 watt right? I would use a 50 watt resistor to have some headroom.

Thanks for your quick response.

 

[Dom65]

Electronic parts from amazon are like a box of chocolates. You never know what you are getting. They sell a lot of fake sub-standard parts. Mouser.com (or Digikey or Newark) is better.

Ok Great. Mouser have Vishays wirewound chassis mount 8 ohm/50 watt resistors.

 

[Dom65]

For a 25W NC@8 ohms, when turned up, expect around 1.77 A RMS, 2.5 A peak, 14.14 V RMS, 40 V peak-to-peak. The data sheet for the resistor will provide information on heat dissipation over a specified time. It should have a heat sink which can fit in a case.

I'll check that.

For amp safety reasons, locking plug would be better than a simple ¼” plug.

So, basically changing the ext amp plug on B15NC and install one for the connection on the pedal casing (with the resistor load inside).

 

[Wasnex]

I'll check that.

So, basically changing the ext amp plug on B15NC and install one for the connection on the pedal casing (with the resistor load inside).

Does the amp switch impedance as shown on the schematic. When you insert a plug into EXT jack it is placed in series with the main speaker and the 16 ohm transformer tap is selected. If the main speaker is not connected, the amp will see open circuit...which is bad.

 

[beans-on-toast]

I'll check that.

So, basically changing the ext amp plug on B15NC and install one for the connection on the pedal casing (with the resistor load inside).

No, you do not have to change the ext amp jack. The ¼” jack is fine. The ext amp jack has a low level signal. It’s function is both a pre-amp out and a power amp in, depending on how you use it. As a pre-amp out, you can use it for recording.

The dummy load resistor should be on the output in place of the speaker. I was simply saying that a good connection is important. If a ¼” plug is used and it comes out, the amp will not have a speaker connected and it can lead to damage. Care must be taken. At least a locking connector like the 4-pin on the speaker cabinet on the box is safer. Of course, some of these amps have been modified to remove the 4-pin cable and replace it with a ¼” jack on the chassis.

 

[beans-on-toast]

Does the amp switch impedance as shown on the schematic. When you insert a plug into EXT jack it is placed in series with the main speaker and the 16 ohm transformer tap is selected. If the main speaker is not connected, the amp will see open circuit...which is bad.

There are two jacks:

EXT SPKR for connecting an external cabinet in addition to the main cabinet. This does the impedance switching.
EXT AMP is the pre-amp out/power amp in that I described in the previous post.

 

[agedhorse]

The pedal casing in which it will be fixed should help dissipate heat. I guess I'm gonna have to test it.

Well that would be for the B15NC which is rated at 30 watt right? I would use a 50 watt resistor to have some headroom.

Thanks for your quick response.

The pedal casing may get very hot.

 

[Dom65]

No, you do not have to change the ext amp jack. The ¼” jack is fine.

My mistake, I meant the main speaker jack (it was modified form the 4 pins to 1/4 inch).

The ext amp jack has a low level signal. It’s function is both a pre-amp out and a power amp in, depending on how you use it. As a pre-amp out, you can use it for recording.

Yes, that's the idea. I recorded a bass part using 3 different ways.
1- Bass to DI to good pre amp (Daking).
2- Bass to B15NC to Fryette power station then from main speaker out (so with power stage include) to pre amp.
3- Bass to B15NC to preamp from the ext amp of B15 (so basically it's pre amp stage only).
I sent the 3 audio files to the owner as a blind test.
His conclusion was the same as mine.
The pre amp only stage (ext amp jack on B15NC) was the best detailed and focus tone.
So that is why I'm looking for a "not too expansive" way of buying or making him an 8 ohm/50 watt load to connect to the main speaker out of the amp while recording the preamp to avoid having to plug a speaker.

The dummy load resistor should be on the output in place of the speaker. I was simply saying that a good connection is important. If a ¼” plug is used and it comes out, the amp will not have a speaker connected and it can lead to damage. Care must be taken. At least a locking connector like the 4-pin on the speaker cabinet on the box is safer. Of course, some of these amps have been modified to remove the 4-pin cable and replace it with a ¼” jack on the chassis.

 

[Dom65]

The pedal casing may get very hot.

Yeah, now looking at some DIY built on the net, I see that they are using some additional heat sinks dissipation.