Beltone AP-A questions

[rhaigh]

Is anyone familiar with the Beltone amp model AP-A or AP-12? Mine came with a 6AR5 power tube but on the internet, I've read users say they have a 6AQ5 power tube in there. Does anyone know what this amp originally came with? A 6AR5 or a 6AQ5? Mine was originally biased to put out around 12W, but the 6AR5 max dissipation is 8.5W. The 6AQ5 max dis. is 12W, so I thought this may have been what the amp originally had in it. Also, these amps came with 8" speakers. Does anyone know what the power handling is of the original speakers? Hard to find much detail about these amps. Thanks!

[Enzo]

What power could the amp make? That much is how much the speaker needs to handle. Doesn't much matter if the original speaker was rated at 15w or 30w.

[rhaigh]

Right, that's what I need to know. What power tube this amp was designed for, therefore the power output.

[Dave H]

What power tube this amp was designed for, therefore the power output.

This schematic shows both tubes. I think it will only put out 3 or 4W max to the speaker with either tube.

[Enzo]

To make my reply more clear, the strongest tube you propose was good for 12 watts, so a 15 watt of more speaker will handle whatever tube you chose. No reason to make it complicated.

As Dave points out, the schematic says use either one. I doubt you'd detect much difference. Both tubes are plugged into the same circuit, and that is what will limit the power output. Just because one tube CAN handle a couple more watts doesn't mean it WILL in this circuit. They considered the two types of tube equivalent. If they ran out of one during production, they could chose to use the other rather than stop production to order tubes.

This is just a very basic guitar amp, there is nothing critical about any of the circuit.

[rhaigh]

Nobody is making this complicated. Its just that I don't think you read my original post at all. lol. I was just looking for info on the original state of this amp. I did not want to know what speaker to put in there. I was wondering if anyone had info on the ORIGINAL speaker. Follow me? As for the tubes, there could quite a difference between using a 6ar5 and a 6aq5. I'm not sure if you understand what biasing is, but these tubes could and should be biased different. Many different tones to be had. I disagree that there is nothing critical about this circuit. The fact that it is so simple, makes almost every part of the circuit critical. By the way, thanks Dave H for the schematic.

[Enzo]

Sorry, I am not trying to be sticky.

WHoever put a speaker in there, be it you, me, or the original designer has to think about power, but this is such a small amp that only the tiniest speakers are threatened, and even then, the most the thing could ever crank out would be your 11-12-whatever watts, so the original speaker would have been in that range. They wouldn't have put 40 watt speakers in it, since they would cost more.

I think I have a reasonable grasp of biasing. This is a single ended amp with a cathode bias, that 420 ohm resistor is going to bias up most anything you stuff in that socket. 420 ohms at 35ma yields about 14v, reasonable for these tubes. The specs on the two tubes are very close. 6AR5 shows a plate max dissipation of 8.5 watts, but look down in typical application, you get about three and a half watts from it. 6AQ5 shsows 12 watts max dissipation, but the typical application shows 4.5 watts. one watt difference, especially at max makes darn little difference.

Let us talk of critical. Look at all the old Fender schematics of the era we hold so dear. On all of them were notes from the factory, and among them was one that said all readings are withing +/-20%. The resistors in them were 20% tolerance parts. The electrolytic caps were far worse at (seriously) specs like -20/+80%. That is what I mean by not critical.

Look at B+ node C, that little 2uf cap. They did that to save money, back then caps were relatively more expensive than today. You can put a 10uf, 20uf, most anything in there and not affect performance much. The 250k plate resistor on the input stage? COuld be 220k, even 100k, amp will work.

You could bypass the 420 cathode resistor with a cap, increase bottom end and some output, but don't, that might make the old speaker flap, but put a ncie modern speaker in ther and it works fine. Every part in it could be changed substantially and the thing will stil work. That is what I mean by not critical.

Now you may feel it sounds better with a 220k instead of 250k, but that is just tone adjustment. You may find through experimentation you prefer the sound with a 500 ohm cathode resistor, or a 270 ohm. Whatever. But again that is tweaking, not criticality. The amp will work within a wide range of parts. That is why with tube amps guys all over the world are running all kinds of different value parts in their amps, and all the amps still work, so clearly it is not critical to the amp that each part be the exact value on the schematic. In the days of the Fender Deluxe, the 100k plate resistor on the first stage could read 80k or 120k and still be a good part within spec.

[rhaigh]

Makes sense. Yeah, I know what you mean by critical now. I was referring to tweaking as opposed to critical operation. Being a user of plexi and bassman type amps for many years, i've recently fallen in love with these little simple amps. So great for recording. I have an interest in them historically as well and thats one of the reason i'm looking for any info on them. These type of amps are cheap, sound great, look great, and are fun to mess with the guts. I really like the sound of it right now, but i'm curious about the Celestion Eight. Inexpensive and I hear good things. I don't know if the magnet can fit in there though. lol

[tbonuss]

This?

[rhaigh]

This is the same as my amp but a different speaker. I've seen this speaker in a couple of online pics of the Beltone AP-A. I've also seen various pics of the amp with the exact same speaker as mine. The funny thing is that my speaker is 4ohm. The speaker in the posted pic says 8ohm. I might have to test the OT to see what the correct speaker impedance should be.

[g1]

I don't think that Mitsubishi speaker and L-pad attenuator in the pic are stock parts.

[rhaigh]

Any advice on a speaker replacement? I dig the sound of this amp but when cranked up, the speaker buzzes badly when playing low notes around B. I've checked for vibrations causing rattling in tubes or chassis but this buzzing is definitely coming from the speaker. Would the celestion eight hold up fine when cranking this amp? The speaker is 15 Watts but I don't have any experience in replacing speakers as my bigger cabs have never failed. The 6bQ5 typically put out around 4.5W clean I think but I assume its putting out more as I am running it turned up past the break up point usually. I could go to a 10" speaker but it would have to be 4ohm.

[The Dude]

First, hook up a larger/higher power/known good speaker to the amp and make sure it's the speaker. There are lots of things that can cause "buzzing on low notes" besides a defective speaker- filter caps, tubes, bias issues, etc., etc.

[rhaigh]

Yeah, I plugged it into my marshall 4x12, cranked it, and it sounded great. No buzzing. I just want something good in there to have a self contained amp so I can bring it around with me. My back can't handle lugging around the marshall anymore . lol.

[The Dude]

Oh, I wasn't suggesting you don't fix it and buy a speaker. I just thought you should make sure it was the speaker that was at fault. It sounds like you already have that covered.