ANOTHER - Impulse buy -- Magnavox 8802

[gadget73]

the sheet conditions above are mostly for fixed bias. the last column is cathode bias though, and it calls for 9K. the ST40 is cathode bias so its likely higher than 6.6K but exactly what they are I have no idea. The ST70 is fixed bias 7591 and those come in at 6.5K.


If they are approx 9K it would be a good fit for a 6V6 though.

 

[Lavane]

I like the s

The 6K6 is cheaper than the 6V6, and is drop-in compatible except tweaks to the cathode and possibly screen drop resistor. It's just an interesting option that may play well enough with the stock 8800 transformers to not have to change them out. (Possibly.) However, if you are committed to using better transformers, and I don't blame you, then staying with the 6V6 is the way to go and try to find something in the 8K - 10K plate to plate.

I built a single ended guitar head amp using a 6K6. The 6K6 sounds good and is a less expensive option to the 6V6. I payed around $5 for the NOS 6K6.

 

[nerdorama]

The Eico ST-40 was cathode biased and used a higher plate impedance. I think they are about 8k if memory serves.

edit: Found the reference. In this post DaveG said he measured the ST-40 outputs at 8250 ohms.

7591A tube comparisons

 

[gadget73]

Seems pretty reasonable then. Might do marginally better if run with fixed bias too since its a slightly lower than datasheet load figure.

 

[S Swanson]

The 6K6 is cheaper than the 6V6, and is drop-in compatible except tweaks to the cathode and possibly screen drop resistor. It's just an interesting option that may play well enough with the stock 8800 transformers to not have to change them out. (Possibly.) However, if you are committed to using better transformers, and I don't blame you, then staying with the 6V6 is the way to go and try to find something in the 8K - 10K plate to plate.

I am - Not at all settled on anything yet - As i mentioned - Just --Is Spitballing the correct word ?- IT seemed Dave was Commenting on the bottle neck of the design- so I went down that rabbit hole ----- My thinking was to (note to self ) Consider all the Options - And as I mentioned --being a novice - there is the comfort of a KNOWN design -

 

[wparks]

the sheet conditions above are mostly for fixed bias. the last column is cathode bias though, and it calls for 9K.

Doh! Good catch. Late night posting . . .

edit: Found the reference. In this post DaveG said he measured the ST-40 outputs at 8250 ohms.
7591A tube comparisons

Excellent data and article. Thank You for posting that!

Those transformers could be really good then- offering good performance and higher power. Sort of big- interesting to see if they would fit on that chassis.
Going with fixed bias would definitely make the most of the limited power supply voltage. But how do we generate the negative supply- Back biasing is out, and no extra taps on the power supply secondary, the second 6.3V winding is better used bucking the primary, so I guess it's either divide down full negative high voltage, or create a voltage multiplier from the main 6.3V filament winding. I'm sure it's been done. Anyone have a good example?

 

[nerdorama]

Bias isn't too difficult to extract from a HV winding. There are several options about halfway down this page.

The Valve Wizard

www.valvewizard.co.uk

 

[gadget73]

yea tapping off one side of the HV supply does the job. I've seen both caps and resistors used for this, and functionally that comes out about the same. Its a fair bit of voltage to drop but bias supplies don't need to be high current so thats not really a problem. Can also do 4 pots for individual bias adjustments if you like.

 

[S Swanson]

Bias isn't too difficult to extract from a HV winding. There are several options about halfway down this page.

The Valve Wizard

www.valvewizard.co.uk

So i mentioned that I have a couple eico (st -40 ) output transformers--Looks like they will work - ( for the 8802 maggie --Prior to that I was looking at a Scott 99d unit (OPT with 6l6's)- The description was (from the seller ) that it was measured @ 10k ohms to the CENTER tap - is that the same as Plate to Plate - I am thinking not-- MY guess is that (and it is a guess ) the transformers Plate to Plate would be -something like 5k is that correct ? - would like some clarification -
I did look on the tube for a explanation - but it was still a bit cryptic - as the math was clear- but did not seem to reference the center tap at all

 

[gadget73]

plate to plate at 10k would measure 2.5K from plate to center tap. Plate to center tap at 10K is 40K plate to plate. I could possibly believe 10K plate to plate but thats not a typical load for a 6L6 family tube. Its vastly more plausible than 40K though. More likely it was a measurement error.

Unless they were checking DC resistance, in which case 10K probably means the transformer is toast. It shouldn't be anywhere near that high.

but you're right, typically we don't care about the center tap. Its the plate to plate load that matters in a push-pull amp.

 

[S Swanson]

plate to plate at 10k would measure 2.5K from plate to center tap. Plate to center tap at 10K is 40K plate to plate. I could possibly believe 10K plate to plate but thats not a typical load for a 6L6 family tube. Its vastly more plausible than 40K though. More likely it was a measurement error.

Unless they were checking DC resistance, in which case 10K probably means the transformer is toast. It shouldn't be anywhere near that high.

but you're right, typically we don't care about the center tap. Its the plate to plate load that matters in a push-pull amp.

Thank you--- seemed like a mystery - and still is --- Just was looking at whatever transformer would work - the ones from the scott 99d seemed to fit - @10k - however I was pointed to the outputs from the 6bq5 family of amps like the the scott 222 or the like - and that got me thinking about the fit (numbers ) of the scott 99d-
Is there of a chance i am trying to make a silk purse out of a sows ear - with the 8802 maggie ?- Kinda on the fence - Jumping the gun - Of course ---as the amp is still on its way

 

[gadget73]

Jumping the gun probably, get it tuned up with the stock iron and the revised circuit first and see how it does. Decide from there if you want to go further.

 

[S Swanson]

Thank you--- seemed like a mystery - and still is --- Just was looking at whatever transformer would work - the ones from the scott 99d seemed to fit - @10k - however I was pointed to the outputs from the 6bq5 family of amps like the the scott 222 or the like - and that got me thinking about the fit (numbers ) of the scott 99d-
Is there of a chance i am trying to make a silk purse out of a sows ear - with the 8802 maggie ?- Kinda on the fence - Jumping the gun - Of course ---as the amp is still on its way

Well that was a long wait - but here it is

 

[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.


View attachment 3750357

View attachment 3750359

Well - I see what you mean about the limitations of the Power supply -- directions - Modifications - ( from Dave ) I am going to do the Mod (The SS rectification) as well as the usual Can-Cap work, and of course the Bucking modification - The idea's seem straight forward -- the sequencing - is a little less clear - Need to think about that --- I did confirm it is operational - it does amplify on both channels - Although very weak --

 

[OBMG74]

Transformers are good: Priceless

For everything else, there is Mastercard!

 

[S Swanson]

Transformers are good: Priceless

For everything else, there is Mastercard!

Yes - Throwing money at a problem will usually get results - Your comment got me thinking that - the very Nature of these Magnavox ( Cheap and Simple ) units ---frees the mind to not worry to much - I mean what's the worst thing that could happen ? Lost time for sure - Perhaps to offset that would be a little more experience ?

 

[wparks]

Well - I see what you mean about the limitations of the Power supply -- directions - Modifications - ( from Dave ) I am going to do the Mod (The SS rectification) as well as the usual Can-Cap work, and of course the Bucking modification - The idea's seem straight forward -- the sequencing - is a little less clear - Need to think about that --- I did confirm it is operational - it does amplify on both channels - Although very weak --

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.

 

[wparks]

I have the full page of the original Sams photofact for the 8800 attached. The picture above is unreadable thanks to AK down-sampling.

 

[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.

View attachment 3763823

Thank you for the reply- - and yes having a way to verify that it still works after any modification - IS a goal - and conformation of the safe things to do is appreciated --- I Looked through my supply of resistors and see - While I do have a 220 ohm 1 Watt - thats all i have so will be ordering those -Question is there a value in that range that has more Utility ? 2watt 3watt you get the picture - OH and the values of the resistors in the power supply - were they calculated with the Original 8802 power transformer ? thanks - steve

 

[wparks]

If I need to kludge together resistors for experiments to walk in exact values before I place a parts order, I usually either parallel or series resistors to cobble together what I need. The cathode resistors for each channel will be dissipating in the ballpark of 1.6 Watts, and you generally want some headroom and stability, which is why 5W resistors are preferred. However, if you have only 1W resistors, you can parallel four 1K 1W resistors and a 2K 1W and be close (222 ohms) etc, or if you have some values a bit lower than 1K you can get close to 220 ohms in the end. (Use the equation for parallel resistors or experiment).

The resistors in the power supply (the 470 ohm and 10K ohm) were the original 8800 values, and Dave kept them as is in his rebuild, so yes, with the original power transformer. (Are you using a different transformer?) The 470 ohm is meant to keep the screen voltage close to the actual plate voltage of the 6V6GT, and the 10K is fairly arbitrary for filtering the pre-amp power supply. I again say 5W for the 470 to make sure the resistor is durable and stable, the Sams has it at 4W, but that was also powering the tuner chassis. Do the best you can there, again paralleling 3-5 1 Watt resistors will get you there safely. For the 10K, a 1W resistor is fine- the preamp pulls very little power.