More Fun With Magnavox: The 9300 Series

Currently there is a set of Pilot EL84 transformers on BT for sale. I'd like to buy them if they would work on my 9300 build. What say ye?

BillWojo
 
Never mind, looks like someone purchased his entire list of OP transformers that were for sale.

BillWojo
 
Good, I was sorely tempted by some of them, but realize my own lack of motivation to actually get around to doing something with the parts.
 
I would like to add a power supply bleed resistor to get rid of dangerous voltage after the amp is turned off.
Would a 500K to 1M work? Would it put excessive load on the power supply or any other bad effect?
scan0003a.jpg
 
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Wouldn't hurt a bit, and it wouldn't be an excessive load. 1M would be 0.38 ma, and dissipate 1/8 watt. A common 1/2 watt part would do the job very well. Double those values for 500K.
 
Harlon -- If you use the design without the buffer stage or the sensitivity reduction network, and install the unit in the console with that configuration, the biggest difference you would notice is that the volume control would tend to react slightly quicker. Theoretically, the console tuner/preamp provides up to 1.0 vac of audio signal, since that is what it took to drive the original stock design of the amplifier to full power. With the modified design as described above, the amplifier only requires .60 vac to develop full power, so you can see that the volume control would tend to react quicker due to the increased sensitivity. Frankly however, it should be fine, as the increased sensitivity won't be that readily apparent unless you're looking for it. I would only install the sensitivity reduction network if you're a stickler for the details of the console's operation.

On the other hand, if you include the buffer stage, it will ensure the very best performance possible, regardless of how the tuner/preamp circuits are designed. The buffer stage allows the amplifier to interact perfectly with any source impedance driving the amplifier -- even if it is a volume control -- which it likely is, almost surely being driven by the output of the volume control on the tuner preamp chassis. If you take this approach, connect the .01 uF input cap directly to the input jack, as well as the 470K resistor shown connected to ground as well. The series 470K resistor between the input jack and .01 uF cap would not be included. In this configuration, the amplifier will have somewhat more gain than the original amplifier did, but again, it just won't be so different as to be of concern.

I hope this helps!

Dave

Dave,
Harlon here. I know this was a while ago, but I’m finally getting around to completing this amp. I’m still looking to incorporate the buffer while utilizing the amp within the existing console. Mostly anyway. The source will be the controller from a voice of music record console, instead of the original Maggie tuner, but same concept.
My question is in the orientation of the .01 uF cap and the 470K resistor. I’ve attached a drawing.
Should the resistor tie in above the cap as shown in the drawing or between the cap and input?

.33467C96-3966-471B-A549-F9F3ACD7D6F2.jpeg

Thanks for the help.
Harlon
 
Harlon -- If you take this approach, connect the .01 uF input cap directly to the input jack, as well as the 470K resistor shown connected to ground as well. The series 470K resistor between the input jack and .01 uF cap would not be included. In this configuration, the amplifier will have somewhat more gain than the original amplifier did, but again, it just won't be so different as to be of concern.

Dave

Hey gadget, the single resistor was a recommendation for my application from Dave last spring. I’m just not sure if I should tie it in upstream or downstream from the cap. From the original schematic I’m thinking it should be between the cap and the input. My drawing shows me tieing it in after the cap.
Thanks,
Harlon
 
ah. It would go across the input jack then, center to ground. No resistor between the .01 and the input jack.
 
Sorry for the repeated dumb questions. I haven’t looked at this stuff in awhile. So one leg of the 470k direct to the center and one leg direct to ground. Am I correct that this part matches the original 470k to ground in the original Magnavox schematic.

Thanks for the help and your patience.

Harlon
 
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Wow, glad to see this thread is still active. What great info! I have a 9302 that I am working on and after reading this whole thread I think I would like to add the floating paraphase inverter modification and also the grid and screen stopper resistors. I may do the EFB next, but it sounds like that is an independent modification that can be done at a later time. My question is which schematic to follow. There have been so many posted.

I assume it is basically the one referenced below (D. Gillespie, Nov 13, 2015), but with grid and screen resistors added, and maybe some change to the NFB. I am using the original output transformers. Is there a more recent schematic that I should follow than this one?
http://audiokarma.org/forums/index....ox-the-9300-series.687735/page-4#post-9213408

Regarding the power supply. Mine (like most I imagine) had the OPT and 6BQ5 screen HV sourced from the same point with no 330R resistor in the power supply as in Dave's schematic. Should I change this (i.e., add a 330R and filter cap) in addition to adding the 100R at each socket screen pin, or just add the 100R's alone?

Also, regarding the input buffer. I have already integrated a tone control/volume preamp into my chassis. I used the exact circuit in the matching Magnavox tuner as shown below. Is it ok to run this preamp circuit directly into DCG floating paraphase without using his buffer circuit? It seems to be working well directly into the OE paraphase now. I don't know if this is a good preamp circuit. I just figured it was good for starters and guaranteed to work since Magnavox designed it to go with the amp. I'd be open to changing it.

I am thinking of the following: OE Magnavox preamp followed by Dave's "Input sensitivity reduction netvork" to get back to 1.0vpp, and then directly into the junction of the 470k and 47k resistors at the input jack of Dave's schematic referenced above (or an updated version). Does this sound right?

Thanks for any help,
Rich

EDIT: I am particularly confused about which value resistor to use as the input to the original 6EU7 (not the buffer). One schematic shows 10K and another 47K. They are both posted by Dave, and there is a comment by Dave that seems to indicate 47K must be used.

Magnavox tone control circuit from console tuner: (There are two small capacitors not shown prior to inputs at R310 and R410 below)
25306704567_61f6f54757_h.jpg
 
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Welcome to AK! -- And thanks for the interest in my modifications posted for the 9300 series of Magnavox amplifiers. A few comments to help with your questions:

1. I would STRONGLY recommend that you implement EFB(tm) when you work on your amplifier. The original design displays boo-coo notch distortion and high THD as a result. The significant difference that the addition of EFB makes cannot be discounted.

2. Add the 330Ω screen grid decoupling section to the power supply and the 100Ω Screen Stability resistors as well. Your tubes will thank you.

3. The schematic you link to is the correct modified version when maintaining the original OPTs. This version was never tested/developed with the buffer circuit added to check for proper HF transient response.

4. It is fine to use the original Magnavox line/tone stage into the modified amplifier, although as the scenario presents itself, the modified amplifier as presented is quite worthy of a proper external quality preamplifier.

5. The modified amplifier is more sensitive. If you want to maintain the original sensitivity as that of the original Magnavox design (particularly since you wish to include their original line/tone stage), then include the Input Sensitivity Reduction Network as shown. It is placed between the input jack, and the junction of the 47K/470K resistors shown.

6. The modified schematic showing a 10K series input resistor is the one developed when using the upgraded Dynaco Z-565 output transformers and input buffer network. The schematic showing a 47K is the one retaining the original OPTs, which you will be using.

Thanks for the interest, and good luck with your project!

Dave
 
Thanks for your reply Dave! This is my first tube amp build, and I can definitely see it is not going to be my last. I am really enjoying this. I will consider the EFB mod in the future. I want to learn more about the science of load lines, etc so I can more fully appreciate the benefits of having a tube biased at precisely the point I want it to be (Yes, I am an electrical engineer :)). The main thing holding me back from fixed bias now is finding a matched set of 4 tubes. These are the only EL84's I have, and they are not too balanced. On my tube tester (Eico 667) I get 86, 91, 96, 103 percent readings.

Here is what I measure installed at idle:

This if the installed order R to L
T93 : T103 : T86 : T91 (Identifying the tube by their performance numbers above)
313 : 310 : 316 : 315 (Plate V)
319 ----------------------- (Screen V)
13.95 -------------------- (Cathode V)
32 : 1292 : 6 : 3 (Grid mV)
35 : 56 : 11 : 14 (plate current ma) (at OPT)

The strongest tube is pulling a of current, and is developing a lot of grid voltage at idle as well.

I see you have significantly higher DC voltages in your design. My original PT died and I am currently using a Hammond 272fx which is rated 300-0-300 @ 268ma. The OEM is also speced as 300-0-300 but seems to be producing a lot higher DC in your case. I wonder how you are seeing 370+ DC under load from the OE PT? I have another original transformer on its way. The Hammond is just a temporary sub.

Does this bias seem normal for the common resistor setup? Would it be improved by splitting into two pairs, each with their own 200 ohm resistor?

Thanks for your help.
 
3. The schematic you link to is the correct modified version when maintaining the original OPTs. This version was never tested/developed with the buffer circuit added to check for proper HF transient response.

I will probably be doing one soon-ish with a buffer and stock transformers so I will post how it works out. Plan is to use this schematic but with the volume control per your suggestion below

More Fun With Magnavox: The 9300 Series



Thank-you Morris. To show the volume control properly, the .01 uF input cap would go to the wiper of the volume control, the low side (min volume) terminal of the control would be grounded, and the high side (max volume) terminal would go to the center pin of the input jack. The two 470K resistors would go away.

If you look at the input circuit as originally drawn, that is the way the two 470K resistors are connected -- with their combined total representing (basically) a 1M pot placed across the input jack, with the .01 uF connected to their junction, which would represent an equivalent level control with its wiper set to mid position if it were a linear control. With the addition of the buffer stage, now any type of attenuation scheme (fixed, variable, stepped, etc.) can be used at its input, and at any setting, without any degradation of audio quality at mid level settings.

Dave
 
The strongest tube is pulling a of current, and is developing a lot of grid voltage at idle as well.

That sounds like either a leaky coupling cap or a bad tube to me. If you shuffle the tubes around, does it do the same thing in another spot or do all tubes in that location do tthe same thing? If it stays with the socket, the cap is probably leaking or the grid resistor is open. If it goes with the tube, I'd make the tube go. They should not develop any significant grid voltage.
 
That sounds like either a leaky coupling cap or a bad tube to me. If you shuffle the tubes around, does it do the same thing in another spot or do all tubes in that location do tthe same thing? If it stays with the socket, the cap is probably leaking or the grid resistor is open. If it goes with the tube, I'd make the tube go. They should not develop any significant grid voltage.

It moves with the tube. The voltage is there even if capacitor is removed. It appears entirely due to very small outward grid current developing voltage across the grid resistor to ground. The tube tests good in my tester (Eico 667). Could it be bad anyway? Why does it draw so much current I wonder? Even if I ground the grid it still draws more current than the others. (Note: the grid voltage I listed above is relative to ground. The grid is still quite negative relative to cathode)
 
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The rectifier tube is also being changed to one with higher output voltage. 5AR4 from the 5U4G.
Ah, that's right. I wonder how much the 5U4 drops. Maybe I'll toss a couple diodes in and see just how much the voltage rises as a test. Thanks.
 
Yes, the tube can be "bad" -- as in, its characteristics have forever been altered. Its symptoms are indicative of a tube that has been operated for significant periods at elevated heater voltage. When that happens, cathode material can boil off of the cathode sleeve and deposit itself onto the next closest thing to it: the grid. This causes the grid to become a secondary emitter when this happens. When it does, the grid draws current through the grid return resistor, causing the notable voltage you observed to appear at the grid terminal, as surely as voltage appears at the cathode terminal when a resistor is connected between it and ground.

Today, vintage console amplifiers are one of the biggest offenders in potentially causing this type of tube damage, as most of their power transformer primary windings were wound to require just 113-114 vac to produce the design center heater voltage of 6.3 vac when operating in the original circuit as designed. I emphasize this last point because today, most vintage console amplifiers do NOT operate as originally designed anymore, because the amplifier chassis is no longer supporting the operation of the tuner/preamp units that were supplied with the vast majority of these units. Rather, they have been modified to operate as a stand alone piece. But without the not insignificant current draw of the tuner/preamp unit, this causes heater voltages to rise even higher than what the simple result of today's higher line voltages would suggest. The result can be a an amplifier with literally 7.00 vac or more supplied to the heaters when operating from a typical 121-122 vac power line, if modified for stand alone operation without appropriate attention given to this concern. This is why rigging the tuner/preamp heater winding available in most Magnavox power transformers for AC line buck duty is so important to the overall health of the unit when it is modified for stand alone operation. Among other things, it prevents the very type of tube damage that has likely happened to the one tube you are concerned about.

I hope this helps!

Dave
 
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