ANOTHER - Impulse buy -- Magnavox 8802

[S Swanson]

Waiting for my New amp ( the magnavox 8802) and thinking that I MAY upgrade the output transformers - thinking of vintage units -- I am seeing some eico st 40 units as well as - the heathkit offerings and what about the Scott's - I would include the dynaco's and perhaps i should - They are commanding a Premium though -
In any case Thought I would ask - so what do you think ? -- Looking at Dave thread - It looks like a upgrade is a recommendation.

 

[Lavane]

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.

 

[nerdorama]

The Eico ST40 outputs are likely very good and at about 8k would fill the bill. MPS has what is apparently a very good version of the Dynaco ST/SCA-35 output. The Heathkit 51-29 and 51-30 outputs are also quite good and are 10k units with 50% screen taps.

 

[gadget73]

personally I'd modify it to get what there is to be had out of the stock iron first and decide if you want to go further from there. The better transformers will benefit from the circuit mods anyway.

 

[Dswankey]

Regarding thread title,

While I don't impulsively buy them, I definitely do not leave them behind if found in the wild at reasonable prices.

My most recent was a beat up console for $50 that upon a little inspection had the 9302 series amp.

Did I need it? no, but for the aforementioned dollar amount I couldn't leave it behind.

 

[S Swanson]

personally I'd modify it to get what there is to be had out of the stock iron first and decide if you want to go further from there. The better transformers will benefit from the circuit mods anyway.

I think your approach is reasonable - Just thinking ahead -- The guy (seller ) said that it worked but was noisy - ,Judging from the work Dave did to Improve the series ( with scope shots ) there is quite a upside in dealing with the deficiencies in stock form -- the square wave was anything but square - really ugly --
The mods seemed like it was worth the effort --

 

[S Swanson]

The Eico ST40 outputs are likely very good and at about 8k would fill the bill. MPS has what is apparently a very good version of the Dynaco ST/SCA-35 output. The Heathkit 51-29 and 51-30 outputs are also quite good and are 10k units with 50% screen taps.

Regarding thread title,

While I don't impulsively buy them, I definitely do not leave them behind if found in the wild at reasonable prices.

My most recent was a beat up console for $50 that upon a little inspection had the 9302 series amp.

Did I need it? no, but for the aforementioned dollar amount I couldn't leave it behind.

Funny - Probably one of reasons (excuses ) for getting the 8802 is I was tired of looking for the 9300 series -
I suppose I should be more Patient -- Passed on one earlier this year -- O-well -
Contentment is not my strong suit -
I understand the 8800 series uses the same chassis as the 9300's -

 

[TomD62]

I had the good fortune to pick up another 9302 and a 8604 for short money at the recent FrankenFest. Since I already did the DG modifications to a 9302, the new one will go on the shelf and the 8604 will get the full DG-SE1 treatment. Looking forward to this project.

 

[S Swanson]

The Eico ST40 outputs are likely very good and at about 8k would fill the bill. MPS has what is apparently a very good version of the Dynaco ST/SCA-35 output. The Heathkit 51-29 and 51-30 outputs are also quite good and are 10k units with 50% screen taps.

I think Daves re-work incorporated the stock values (measured @ 10 K primary ) from the original output transformers - if i remember correctly --- SOOo does that open up the transformer options ?
I do see some Scott output transformers -- for the 7591 tube set -- for sale - should i also include that option --- also -- how about the Heathkit 51-58's ---

I had the good fortune to pick up another 9302 and a 8604 for short money at the recent FrankenFest. Since I already did the DG modifications to a 9302, the new one will go on the shelf and the 8604 will get the full DG-SE1 treatment. Looking forward to this project.

 

[S Swanson]

I had the good fortune to pick up another 9302 and a 8604 for short money at the recent FrankenFest. Since I already did the DG modifications to a 9302, the new one will go on the shelf and the 8604 will get the full DG-SE1 treatment. Looking forward to this project.

listening to my 8600 maggie - Dave's design is marvelous - a very nice project - People seemed genuinely shocked at the performance - I will take exception to one of the things Dave said though --- You do not need to get a set of very sensitive speakers to enjoy this - Amp -- Just Plug it in to what you Have - My set up is not very sensitive but sounds fantastic

 

[wparks]

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.

 

[gadget73]

I have wondered what you might be able to get out of one of these with 6K6 tubes. Those work with a higher plate to plate load, so in theory the stock transformers should be less of a mismatch than with 6V6 when run to an 8 ohm load. Similar voltages and current otherwise so the power supply would be fine with it. Probably needs a different cathode resistor to set the output tube idle point but thats a minor detail. Not sure it would actually perform markedly better than 6V6 but if you had one that came tubeless or with bad tubes, it might be something worth exploring.

 

[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

Thanks Warren - I was planning to steal from your Mirror Mite design --- still may -- And yes there is a method to my madness - as i am a AMATEUR - ( the french definition -- A LOVER OF -) AND the implied description-- a novice ---
Looking at the offerings of (Vintage )-Output transformers-- I see many with a 6.5k primary and some with 7.5 are these OK to use ? - I think the 10k that Dave measured was a surprise even to him.

 

[S Swanson]

I have wondered what you might be able to get out of one of these with 6K6 tubes. Those work with a higher plate to plate load, so in theory the stock transformers should be less of a mismatch than with 6V6 when run to an 8 ohm load. Similar voltages and current otherwise so the power supply would be fine with it. Probably needs a different cathode resistor to set the output tube idle point but thats a minor detail. Not sure it would actually perform markedly better than 6V6 but if you had one that came tubeless or with bad tubes, it might be something worth exploring.


View attachment 3750398

Looks interesting - perhaps when I am better schooled -- I do like the option of the less expensive tubes - and of the Surprise nature of the design -- potential ---
Please indulge a Analogy --- If one thinks of Tubes as a SPICE and the designer as a COOK - the process can be quite a adventure - And the outcome ? well that is- the Surprise -
As a example in cooking I tried Sumac ( a unusual spice ) With onions garlic and chicken - it is a real flavor bomb-- But you don't know until -------------

 

[wparks]

I see many with a 6.5k primary and some with 7.5 are these OK to use ? - I think the 10k that Dave measured was a surprise even to him.

Since that is a push pull transformer, I assume those numbers are plate to plate? Dave was not surprised to see the transformers were 10K- that is pretty much standard for both the 6V6GT and the 6BQ5- What he was saying is that the Sams Photofact had it wrong by indicating they were only 6K plate to plate. He goes on to say that the 10K plate to plate "is entirely correct for this application."

When you consider output transformer impedance for a given tube, there is a chart found in the back of some datasheets- The one below is reproduced from the GE datasheet for the 6V6GT from Frank's pocnet. This is for single ended, at plate = screen = 250V (to cathode) and shows that output power versus transformer impedance. You can see that the output power is maximum from about 5K to 9K, and distortion dips to a minimum around 6K ohms. Since this is for single ended, multiply that by two to get the plate to plate impedance seen while in class A (lower output levels). This is why for the 250V/250V to cathode voltage ballpark, 10K plate to plate (5K single ended) is generally preferred, but if you had to diverge from 10K going slightly higher can be better at that voltage. Using 6.5K or 7.5K plate to plate I think would be quite disappointing- (which would put you at the 3.2K to 3.7K area on this chart- better to stick with 10K at least. However . . .



There is another portion of the datasheet that seems to contradict this- In the chart below, notice that for higher voltage, plate = screen = 285V/285V (to cathode, the voltage you happen to be running at) that they recommend a lower 8K plate to plate primary- allowing you to get 14W output at even =lower= distortion. (But keep in mind 14W is spec'd at the plate, only about 80% of that makes it through the output transformer to the speaker.) General consensus is that a lower impedance transformer will allow you to generate more output power (within limits). So considering you are running at that higher 285V voltage, you could go as low as 8K plate to plate and probably make out really well. In the end, the actual transformer plate to plate impedance is usually a lot more about what you can find that is available and in your budget, and meets all of your other needs too (size, appearance, output taps, performance level) etc.
Whatever you decide on, (I'd shoot for 8K to 10K plate to plate) will work just fine.

 

[wparks]

I have wondered what you might be able to get out of one of these with 6K6 tubes. Those work with a higher plate to plate load, so in theory the stock transformers should be less of a mismatch than with 6V6 when run to an 8 ohm load. Similar voltages and current otherwise so the power supply would be fine with it. Probably needs a different cathode resistor to set the output tube idle point but that's a minor detail. Not sure it would actually perform markedly better than 6V6 but if you had one that came tubeless or with bad tubes, it might be something worth exploring.

That is a really interesting idea. 6K6 tubes can be had for really cheap on Ebay. (No affiliation). They would be drop-in pin compatible. The bias currents run pretty low, so you would even get some extra B+ voltage from the lighter loading. (Right- we would need to tweak the cathode and maybe the screen resistors).

From the very similar single ended at 250/250V chart for the 6K6, the sweet spot is extended from 6K to 10K, so the 8800's 10K : 4 ohm transformers (becoming 20K : 8 ohms) would probably work really well. Even better considering many speaker are more like 6 ohms these days. It would be interesting to see if the 8800 would make more power using 6K6's or 6V6's with the stock iron. I wonder if Dave G considered that- Might make a great build thread experiment for someone rebuilding an 8800 . . .

 

[S Swanson]

Since that is a push pull transformer, I assume those numbers are plate to plate? Dave was not surprised to see the transformers were 10K- that is pretty much standard for both the 6V6GT and the 6BQ5- What he was saying is that the Sams Photofact had it wrong by indicating they were only 6K plate to plate. He goes on to say that the 10K plate to plate "is entirely correct for this application."

When you consider output transformer impedance for a given tube, there is a chart found in the back of some datasheets- The one below is reproduced from the GE datasheet for the 6V6GT from Frank's pocnet. This is for single ended, at plate = screen = 250V (to cathode) and shows that output power versus transformer impedance. You can see that the output power is maximum from about 5K to 9K, and distortion dips to a minimum around 6K ohms. Since this is for single ended, multiply that by two to get the plate to plate impedance seen while in class A (lower output levels). This is why for the 250V/250V to cathode voltage ballpark, 10K plate to plate (5K single ended) is generally preferred, but if you had to diverge from 10K going slightly higher can be better at that voltage. Using 6.5K or 7.5K plate to plate I think would be quite disappointing- (which would put you at the 3.2K to 3.7K area on this chart- better to stick with 10K at least. However . . .

View attachment 3750678

There is another portion of the datasheet that seems to contradict this- In the chart below, notice that for higher voltage, plate = screen = 285V/285V (to cathode, the voltage you happen to be running at) that they recommend a lower 8K plate to plate primary- allowing you to get 14W output at even =lower= distortion. (But keep in mind 14W is spec'd at the plate, only about 80% of that makes it through the output transformer to the speaker.) General consensus is that a lower impedance transformer will allow you to generate more output power (within limits). So considering you are running at that higher 285V voltage, you could go as low as 8K plate to plate and probably make out really well. In the end, the actual transformer plate to plate impedance is usually a lot more about what you can find that is available and in your budget, and meets all of your other needs too (size, appearance, output taps, performance level) etc.
Whatever you decide on, (I'd shoot for 8K to 10K plate to plate) will work just fine.

View attachment 3750679

Thanks for the correction - I remembered his comment - (contrasting the actual measurement with the stated schematic ) and thought that his development would be Different --with the measured value; verses the incorrect schematic Notation, - what I did not digest was -the - Affirmation of the 10 k value - my Bad --
A update - I had a vague memory of buying some output transformers - went through my stuff and Vole ! there is a set of Eico ST 40 outputs - Was i drinking or something - could not recall the purchase - but here they are - I am guessing i should measure them at some point - Just to be sure --

 

[S Swanson]

Since that is a push pull transformer, I assume those numbers are plate to plate? Dave was not surprised to see the transformers were 10K- that is pretty much standard for both the 6V6GT and the 6BQ5- What he was saying is that the Sams Photofact had it wrong by indicating they were only 6K plate to plate. He goes on to say that the 10K plate to plate "is entirely correct for this application."

When you consider output transformer impedance for a given tube, there is a chart found in the back of some datasheets- The one below is reproduced from the GE datasheet for the 6V6GT from Frank's pocnet. This is for single ended, at plate = screen = 250V (to cathode) and shows that output power versus transformer impedance. You can see that the output power is maximum from about 5K to 9K, and distortion dips to a minimum around 6K ohms. Since this is for single ended, multiply that by two to get the plate to plate impedance seen while in class A (lower output levels). This is why for the 250V/250V to cathode voltage ballpark, 10K plate to plate (5K single ended) is generally preferred, but if you had to diverge from 10K going slightly higher can be better at that voltage. Using 6.5K or 7.5K plate to plate I think would be quite disappointing- (which would put you at the 3.2K to 3.7K area on this chart- better to stick with 10K at least. However . . .

View attachment 3750678

There is another portion of the datasheet that seems to contradict this- In the chart below, notice that for higher voltage, plate = screen = 285V/285V (to cathode, the voltage you happen to be running at) that they recommend a lower 8K plate to plate primary- allowing you to get 14W output at even =lower= distortion. (But keep in mind 14W is spec'd at the plate, only about 80% of that makes it through the output transformer to the speaker.) General consensus is that a lower impedance transformer will allow you to generate more output power (within limits). So considering you are running at that higher 285V voltage, you could go as low as 8K plate to plate and probably make out really well. In the end, the actual transformer plate to plate impedance is usually a lot more about what you can find that is available and in your budget, and meets all of your other needs too (size, appearance, output taps, performance level) etc.
Whatever you decide on, (I'd shoot for 8K to 10K plate to plate) will work just fine.

View attachment 3750679

Is this version - The 6k6 interesting enough to try ? I am not married to the 6v6 tube Idea - it seems well developed though-- would you want to see it (the 6k6 ) built ? -- I can paint by the numbers, but that's about all I can offer ,and off course various parts - (some i don't remember ) A community development project perhaps ? anyone want contribute ?--
( thoughts that is - expertise ) Could be fun

 

[wparks]

went through my stuff and Vole ! there is a set of Eico ST 40 outputs

I wish when I woke up from a night of drinking I could find high-end output transformers . . .

The Eico ST-40 used 7591A output tubes- if you look at the datasheet for the 7591, for class AB1 push-pull, for all but the highest voltages it recommends a plate to plate load of 6.6K. These are probably going to be too low to use with the 6V6, and WAY too low to use with the 6K6. I recommend saving those for a push-pull amp with more like a 6L6 type tube. Want to sell them?

 

[wparks]

Is this version - The 6k6 interesting enough to try ? I am not married to the 6v6 tube Idea - it seems well developed though

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.