More Fun With Magnavox: The 9300 Series

[gadget73]

1.25 volts? Thats a vast amount of current. It should be something like 0.44 per pair. Leaky coupling caps or bad output tubes sounds likely here. Pull the output tubes and see if the voltage comes back up.

 

[Morrism]

I installed the new Russian tubes. The voltages came up. Yay!
I checked the bias and got 10-10-70-40 mV. The high bias didn't change when I swapped tubes.
I checked my wiring and found I had connected the feedback from the 8ohm output to the wrong side of the resistor and parallel cap, which bypassed them.

After fixing the wiring, the PS voltages are now too high. I get these measurments:
AC line - 117VAC (variac)
PS voltages - 414, 412, 353 VAC (these should be 375, 370, 320)
Bias - 18.9, 18.7, 14, 21.8 mV

 

[dcgillespie]

Did you connect the extra heater winding to act as a buck winding as shown on the schematic?

Dave

 

[Morrism]

Did you connect the extra heater winding to act as a buck winding as shown on the schematic?

Dave

Not yet. I'm using a variac to set the AC line to 117. My AC line varies from 120 to 124, So I will buck the heater when I get everything else worked out.

 

[gadget73]

Between that and a bit more bias you should have it. More voltage across those cathode resistors means more load on the supply.

 

[Morrism]

So 414V B+ is OK?
I'm going to do the one at a time tube bias check to find a set that matches.

 

[kirk57]

That's correct. Ideally it is good to get the two channels within at least 10% of each other, which you're a ways away from.

One of the excellent things you can do now that you've installed EFB(tm), is to use the amplifier to match the tubes for you in a way that you never could before. This is because with EFB installed, that amplifier can operate safely with just one output tube installed -- which could never be done with the stock design, as it would burn up the tube. To match your tubes:

1. Remove all four tubes, and select one tube socket to use as the test socket.

2. Install the first tube to test in the socket, and with your meter connected across the appropriate 10 Ohm resistor, adjust the bias control for .22 vdc on your meter. Once stabilized at that reading, record it as the value for that tube.

3. Now, power the amplifier down, remove the first tube tested, and without touching the bias control, install the second tube to be tested into the same socket. Turn the amplifier on, and record the reading that the tube stabilizes at.

Proceed with the same process for all tubes to be tested. In this way, you can match your tubes rather closely under quiescent condition. Once you find four that have readings within no more than a 10% difference (5% is really good), then you've produced a set of tubes that is much more closely matched, than if you just shoot in the dark.

I hope this helps!

Dave

Just spent some time testing all my EL84-ish tubes. They were all over the place. The closest I'm getting to a 'matched quad' is actually pretty danged close, within .005 volts. The thing is, one of them is a Magnavox 6BQ5 (made in Germany: Tele?) two are newish JJ EL84 and one is a Russian 6n14 EB. I'm guessing that the tubes matching is more important than the name on the side? With this quad I get 0.44 volts on one side and 0.45 on the other. Pretty good.

It's like the UN in there with all those countries represented

 

[gadget73]

So 414V B+ is OK?

I'd expect it to come down once all the tubes are biased properly. Looks like its set a little low currently, so the voltage will be high.

 

[Morrism]

I'd expect it to come down once all the tubes are biased properly. Looks like its set a little low currently, so the voltage will be high.

I have the V2 amp with 4 separate test points. If I remember correctly, they are supposed to be set to 20mA on each tube.

 

[Morrism]

I tried a "matched set" of Tung-sol" EL84 re-issue.
The B+ voltage was 395VDC. The bias mesured 74, 75, 93, 70 mV. The adjustment pot ran out of travel so wa unable to adjust any lower.

 

[bberkom]

I have a 9300 with the EFB and bucking circuit mods and stock output transformers that I want to use more regularly with my 16 ohm Sony speakers. I've used this combo briefly in the past with good sonic results, but I'm worried what the impedance mismatch may be causing electrically. Since the speakers are 98db sensitive, the amp is not stressed at my listening levels despite the loss in output power from increasing the impedance. From doing some research, I understand that the OPT's on the 9300 are meant for a nominal 4ohm load and that increasing the load could put additional strain on the screen grid and have an effect on plate voltage of the output tubes.

So I know I'll see some loss of power and maybe tonal differences that can be caused by mismatched impedances, but will running the 16 ohm speakers on a 4 ohm tap have any other long-term negative effects on the amp besides possibly wearing out the tubes more quickly as long as it's prevented from clipping? Or should I simply avoid using 16 ohm speakers with this amp? It's just such a nice-sounding combo.

 

[punkzter]

I hope I'm correct...but it's been my understanding that moving up is not the problem. If the OPT's are rated for 4 ohms, you can drive 4, 8, or 16 ohm speakers. But you wouldn't want to move in the other direction (i.e. driving 4 ohm speakers with an 8 ohm OPT).

 

[bberkom]

I hope I'm correct...but it's been my understanding that moving up is not the problem. If the OPT's are rated for 4 ohms, you can drive 4, 8, or 16 ohm speakers. But you wouldn't want to move in the other direction (i.e. driving 4 ohm speakers with an 8 ohm OPT).

Yep that is my understanding as well. I'm just curious if doubling the impedance twice will have adverse effects outside of the loss of output power, causing concerns with the tubes or other components in the amp.

 

[Shadowdog]

Tube amps are pretty tough as I was driving 4 ohm speakers with the 8 or 16 ohm (where it sounded better) and it never hurt over a few years of even loud playing!

 

[gadget73]

Running a 16 ohm speaker off a 4 ohm transformer won't really do it any favors. its not likely to blow it up, but output power will be considerably reduced and the distortion will go up a fair bit as well. Its also not real easy on the screens either. They will probably be taking some extra abuse since the plate load is such a high impedance.

 

[GordonW]

I'd be more worried about the screens, than the OPTs.

As long as the load is even remotely in any sort of ballpark, you won't get inductive spiking in the transformers It doesn't take a huge amount of load to damp that out.

OTOH, you may run into what I had happen with a Scott 299D lately (headphone jack/speakers off load wasn't sufficient)- it may start killing marginally-robust tubes, ostensibly by overloading the screens. Tubes that are more "overkill" in their construction may still be OK, but those that barely meet spec may not...

Regards,
Gordon.

 

[bberkom]

All in all, it doesn't seem like a great idea then. I'll probably avoid that combination then since I have other options. Thanks for the input.

 

[GordonW]

Yeah, 8 ohms is fine (the original load was a :"light" 4 ohms), but 16 ohms is probably pushing it, with the stock OPTs, if it's played at higher volume levels..

Regards,
Gordon.

 

[Harlon]

The original OPTs were 7600 Ohm units, while the Z-565s measure 7800+ Ohms. Not a big deal, but enough to change the output slightly.

Dave

I'm finally getting around to finishing my 9304-20, I've decided to incorporate the Dave's modifications to my rebuild.

As far as upgrading OPTs. Edcor offers a 7600 Ohm OPT. In theory, could you sub these in for the original stock units without needing a scope to reset the NFB resistor values. Plug them into Dave's "modified" version to get the advantage of his improvements and the improved bandwidth, etc,..., of th newer OPTs.

 

[GordonW]

I'm finally getting around to finishing my 9304-20, I've decided to incorporate the Dave's modifications to my rebuild.

As far as upgrading OPTs. Edcor offers a 7600 Ohm OPT. In theory, could you sub these in for the original stock units without needing a scope to reset the NFB resistor values. Plug them into Dave's "modified" version to get the advantage of his improvements and the improved bandwidth, etc,..., of th newer OPTs.

You are basically correct about the feedback resistors (the feedback ratios)- with the caveat that they may need different frequency or phase compensation (due to different resonant behavior, different inductance, and such). Even if the resistors don't need to be changed, it may need a different phase advance cap, step cap/resistor, Zobel, or whatnot. That would have to be experimentally determined, for peak performance.

Regards,
Gordon.