ANOTHER - Impulse buy -- Magnavox 8802

[gadget73]

I don't know that its overly critical, tube amp voltages are often 15% or so tolerance anyway so as long as you're in the neighborhood it won't care.

 

[S Swanson]

I suggest giving it a haircut first, by removing the molex connector and the unnecessary wires that dead-end at that connector. (You have already jumpered the AC power to make it work.

The bucking of the primary can be done by itself at any time, is always beneficial under any circumstance, so I recommend doing that first.

Next, Since you are going to be switching to solid state rectification, I suggest you do the work to prepare for the higher voltages first.

First, the 6V6GT output tube cathode resistor (which is shared across =all 4= of the output tubes) is sized based upon those original voltages. If you increase this voltage without increasing this resistor, your output tubes will pull =WAY= too much power. The original cathode resistor (from the Sams) is listed as a single 180 ohms. Based upon Dave G's final version, and rebuilds I have done, I recommend starting with a 220 ohm (at 2W to 5W), one for each channel (The two left cathodes and another for the two right cathodes.) Once you have done that, the output tubes are safe for all power supply modifications to come. The original single 180 ohm cathode resistor was bypassed with a single capacitor from the can. Following the suggested schematic, you will want to bypass each 220 ohm cathode resistor for each channel with a 100uF 50V cap at least, and abandon the 30uF cap inside the can.

The second thing you need to do to prepare for higher voltages is replacing the electrolytic capacitors- you will see even higher supply voltages when you do this, so it's wise to have new caps in place before you do. The values listed in the power supply schematic I posted earlier (from Dave's rebuild) will do very nicely. Might as well throw the choke in there as well.

Final step would be to replace the 6CA4 with the 1N4007 or similar diodes. You will want to add a new terminal strip to mount the diodes to and be sure to give them a little space as this is the highest voltage.

The amplifier can be tested after each of these steps, to make sure everything remains functional.

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Well I have made some progress
-cleaned out some of the old stuff ( Molex plug and other ) - got rid of the tube rectifier - installed the Diode's = a couple tag strips --- re stuffed the Can Cap - installed a Choke, and new switch (old one was ugly ) and started wiring the new power supply - Lets see O--- new star ground and pre - installed ground wires --- Question though about the 180 ohm sand resistor ( common cathode resistor ) I cut it out --BUT only have 220 ohm 1w resistors == Today will they ( 2 for each set of tubes ) be adequate - at least to start it up again -- to make sure it still works - and to check voltages ?

 

[gadget73]

They will work but they will get hot and may discolor slightly. Not entirely sure the power dissipation on these but its probably pretty darn close to 1 watt.

Looks like the revised schematic calls for one 220 ohm per-pair of tubes, not two for each set.

 

[S Swanson]

They will work but they will get hot and may discolor slightly. Not entirely sure the power dissipation on these but its probably pretty darn close to 1 watt.

Looks like the revised schematic calls for one 220 ohm per-pair of tubes, not two for each set.

sorry for the confusion - I was thinking one per pair of tubes - I need to buy some stuff so there is no rush - thank you though -

 

[wparks]

If we assume a worst case cathode current of 40mA per 6V6, that's 80mA per channel, through 220 ohms that's 1.4 Watts. 2W was barely adequate in the schematic, I think I will bump that up to indicate a 5 watt ceramic resistor. If you =only= had 220 ohm resistors, sounds crazy, but you could use four- two in parallel in series with two in parallel (or two in series parallel another two in series) if that makes sense. Makes a 220 ohm 4 watt resistor.

 

[gadget73]

yeah if its dissipating 1.4 watts, I'd say 3 watts would be a comfortable minimum rating there. 4-5 would be better.

 

[S Swanson]

If we assume a worst case cathode current of 40mA per 6V6, that's 80mA per channel, through 220 ohms that's 1.4 Watts. 2W was barely adequate in the schematic, I think I will bump that up to indicate a 5 watt ceramic resistor. If you =only= had 220 ohm resistors, sounds crazy, but you could use four- two in parallel in series with two in parallel (or two in series parallel another two in series) if that makes sense. Makes a 220 ohm 4 watt resistor.

View attachment 3769995

Order came in so Ihave some 220ohm 5 w resistors -- forward is always good - Kind of a stupid question though - the 10 k resistor - on the impute there is a arrow pointing to it,- from the center of the 470 k and 33k are they connected ?

 

[gadget73]

Thats a volume pot. The arrow is the wiper. If you don't have a volume pot, the junction of the 33K and 470K go to the input jack.

 

[S Swanson]

Ok (rolling up sleeves . . .) The 8802 is an amp that will require significant mods (more than the 8600, fewer than the 9300) to correct a myriad of problems from Magnavox- I think this is a great next step for your progression- having rebuilt a stock 8600, a "Twisted Sister" 6AQ5 based custom 8600 style variant, and hoping in the future to rebuild a 9300- The 8800 is the perfect "stepping stone" in that journey.

At a high level, you will want to do away with the puny 6CA4 rectifier- it was excellent for the low-power 8600 design, but has no place on a stereo push-pull amp- it's running at it's limit, lowers an already too-low high voltage, and causes bad sag and droop in the power supply. Conversion to diode rectification, and the addition of a filter choke will be required for any sort of decent performance.

The two problems that I have with Dave G's writeup on the 8800 is that he never posts a schematic of the modified power supply (It's easy enough to glean from the text, but it's a large missing piece for those just looking for a recipe) and he included adjustable bias, DC current balance, and AC distortion balance controls, mostly to compensate for the sub-par output transformers. These controls are all very nice to have, but significantly complicates an otherwise straight-forward design, and requires extra work and opportunities for mistakes to unroll those mods from the one (stock OPT) schematic that is given. Again, this is covered in the text, but not in schematic form.

I built a push-pull 6AQ5 amp, "The Mirror Mite", which was based heavily upon Dave's 8800 redesign. It incorporated the bias adjustment and DC current balance, and it sure sounds nice, and I like the controls, but they do significantly increase the size and complexity of the build.

I recently helped a friend who wanted to rebuild an 8800 without those controls, and I took the time to draw up a nice schematic of one channel with Dave's recipe without those controls, and the modified power supply, shown below. The cathode resistor and feedback cap are tentative, and can be adjusted to fine tune the amp to your particular voltages and output transformers.

As far as output transformers- Anything in the roughly 10K plate to plate impedance will work well. The stock iron is pretty meager, and almost anything is a significant improvement, especially an 8 ohm output. 6V6GT push-pulls sound mighty nice, and better iron is a worthwhile investment here.


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Looks like i Missed ordering the --- .01- 1400 v CAP?--- on the impute-- It is not obvious what this for care to elaborate ? Safety ? cap? what is it --a Electrolytic or other ?

 

[gadget73]

are you talking about the power line to chassis or across the line cap? I don't remember offhand which type this uses. Those can be replaced with an XY type safety cap at 250vac.

original is probably ceramic.

 

[knockbill]

The smaller 15W Edcore's with end bells are supposed to sound nice.
I used a pair of the 10 watt versions in a sweep tube amp and I like them.
The responce is 40hz to 18khz for both the 10 and 15 watt.
Stephie used the 15W in an amp build and liked them.

+1....

 

[S Swanson]

are you talking about the power line to chassis or across the line cap? I don't remember offhand which type this uses. Those can be replaced with an XY type safety cap at 250vac.

original is probably ceramic.

This is the drawing --- Its a mod by WPARKS -- the cap is the one on the 120 vac -side of the P-transformer just after the switch

 

[gadget73]

Across the line cap. I'd still use a safety cap there. Those are specifically rated for that sort of use. That one in particular is an X type, but I usually just get the XY kind so they work in either application. The original cap is probably 1.4KV.

 

[wparks]

Yep- Magnavox put this cap in long before safety caps existed, and they just used a ceramic rated to 1400V. If you do this, either re-use the original Magnavox 1400V cap (that was typically in these amps), or, you can just do without it, or best, as @gadget73 says, get an XY safety cap- You could use one like this, or just about any one with an X2 or X1 rating at higher than say 400V or so. Here is a representative picture:

 

[S Swanson]

Thats a volume pot. The arrow is the wiper. If you don't have a volume pot, the junction of the 33K and 470K go to the input jack.

I don't have a volume pot - Should I get one ? it seems reasonable to have something though -For my other systems-- I have a nice pre-amp for the cd and vinyl stuff - - the other S E Ps are connected to a (wiim) so the volume is remotely accessed - What are the the options though for a volume pot ? seems like a nice option if it does not degrade the sound -
I think I remember a pot value ??? for the DGse1 was it ( 100 ohm perhaps ?) anyway--
AS this is for the Maggie 8800 series, I am not sure-- quality wise (OPTION's ?) also - a bit in the dark

 

[nerdorama]

was it ( 100 ohm perhaps ?)

Probably 100k.

 

[gadget73]

If you want to use it directly behind a source it would be useful but if you are using it with a preamp there really isn't any need for it. Those parts are not on Dave's original schematic, its something @wparks added to his.

Also 10K may or may not be the value to use. Its a pretty low value honestly, unless your source has a lot of drive ability it may need to be higher. 50K-100K would be more typical.

 

[wparks]

Well, depends upon the source. The only source I believe that would have trouble with 10K would be a vacuum tube pre-amp. Virtually any modern source, or line stage, or pre-amp can drive 10K like it's not even there. Most op-amp and transistor output stages these days can actually drive headphones directly, so have an output impedance usually well under 1K, often well under 100 ohms. I chose 10K for the best noise immunity. I think 100K is a great compromise if you are worried about it.

@S Swanson - If you do add a volume pot, try to make it a decent ganged logarithmic pot. I tend to be a cheapskate, and I have found that these pots from Amplified Parts (offered in both 10K and 100K and well as others) are plenty good. Many would seek better, some would seek much better.

 

[S Swanson]

Well, depends upon the source. The only source I believe that would have trouble with 10K would be a vacuum tube pre-amp. Virtually any modern source, or line stage, or pre-amp can drive 10K like it's not even there. Most op-amp and transistor output stages these days can actually drive headphones directly, so have an output impedance usually well under 1K, often well under 100 ohms. I chose 10K for the best noise immunity. I think 100K is a great compromise if you are worried about it.

@S Swanson - If you do add a volume pot, try to make it a decent ganged logarithmic pot. I tend to be a cheapskate, and I have found that these pots from Amplified Parts (offered in both 10K and 100K and well as others) are plenty good. Many would seek better, some would seek much better.

Well Happens I have several 1 - all tube and a SS - I suppose some flexibility is nice - sounds like there are many options --- or use none at all

 

[wparks]

If you want to use the amp with both the SS and tube pre-amps you may consider just leaving the volume pot out. When using a pre-amp I always just leave the volume pot all the way up, but in this case it will still unduly load the tube pre-amp.