More Fun With Magnavox: The 9300 Series

[rjpjnk]

 

[rjpjnk]

Maybe this will help. I made a couple plots.

The first one is a 10 volt scaled sine with 10 volts dc offset added: 10 + 10*sin(wt)
This has RMS = 12.25

The second one is a full wave rectified 20 volt sine wave: 20 * abs(sin(wt))
This has RMS = 14.14

Both are 0 to 20 volts. Is your first drawing one of these?

 

[rjpjnk]

 

[rjpjnk]

 

[Tubeglowpio]

Can you run a 12v bulb from a 24v peak FW rectifier?

If you have a virtual ground 12 volts up you should be able to. Otherwise your bulb won't last as long running on about 16-17 volts rms.

 

[rjpjnk]

 

[rjpjnk]

If you have a virtual ground 12 volts up you should be able to. Otherwise your bulb won't last as long running on about 16-17 volts rms.

Correct. The RMS of this waveform is 16.8, so not good for 12v bulb life.

 

[rjpjnk]

 

[rjpjnk]

I did a little searching for a technical discussion that was not too crazy mathematical but I couldn't find anything great. However, this article is pretty good, and does address specifically the issue of a sin wave with DC offset as in your first drawing. It shows how to calculate the RMS and how to identify the offset (a.k.a. DC freq component).

Maybe this will help.

https://masteringelectronicsdesign.com/how-to-derive-the-rms-value-of-a-sine-wave-with-a-dc-offset/


... much equations later...

 

[dcgillespie]

Thank-you!

 

[rjpjnk]

Great idea! I just went through the posts, and everything from Post 703 until this one can be moved with the exception of the following posts, which are relevant to the DG9300 thread

704, 706, 707, 710, 711, 713, 715, 718, 720. I would keep these here Mod.

 

[rjpjnk]

I swapped the rectifier tube to 5AR4 and also implemented the SS diode, plate bridge, and cap mod that Dave discussed on the ST-120 thread. Now the tube is only there to delay the turn-on time and provide some resistance. Seems to work well like this and I get a solid 16 Watts, but I am wondering if the voltage is too high for this circuit.

At idle (20ma cathode current) I measure around 390 at the OPT CT, and 385 on the 6BQ5 plates. I think the 6EU7's plates were about 185-190v.

EDIT: Here is the link to the thread mentioned above:
http://audiokarma.org/forums/index....120-wont-hold-bias.657410/page-2#post-8760949

 

[gadget73]

Subtract out whatever voltage you've got at the cathode to get effective plate voltage. Its probably closer to 365 at that point. Even at 385, with 20ma per tube you're under 8 watts plate dissipation. EL84M or 7189 tubes would suffer that sort of voltage. The current is low to the point of allowing the tubes to last just about forever.

 

[Morrism]

I have a working 9302 built to the original schematic dated Nov 13, 2015, which uses the stock OPT's.
I want to upgrade the OPT's to the Dynaco Z-565 with as little parts changing as possible. Specifically, can I change the transformers without adding the extra 6EU7 buffer?

 

[kirk57]

Note: the schematic dated Nov 13 2015 is at the end of post #75 in this thread.
I’m not one of the experts here but I think you only need the buffer if your preamp situation is like what Dave describes in post #388.

 

[rjpjnk]

I have a working 9302 built to the original schematic dated Nov 13, 2015, which uses the stock OPT's.
I want to upgrade the OPT's to the Dynaco Z-565 with as little parts changing as possible. Specifically, can I change the transformers without adding the extra 6EU7 buffer?

I just did the exact same, so I can help. There are a total of 20 resistors and capacitors that need to be changed to go from the original DG design to the version with the updated OPTs. Just get the schematic for each and compare. Feedback loop, step filter, cathode bypass, and grid resistors are different.

As mentioned above, you do not need the buffer unless you intend to add attenuation (vol control, etc) to the amp.

EDIT: Here is an updated pic of my amp with the Dynakit OPTs and new rectifier tube. And I found a really cool Magnificent Magnavox tin label too. There's not much left that is Magnavox, but it reminds me of its roots. I think this is its final state now. Moving on to ST70 build next.

 

[ArchieType]

WRAP UP -- Part 1...
Comment: Because this is a development model, the original good can cap is still in place and in use, providing good service. As a result, the 10 uF section originally used to decouple the phase inverter stage is now used to decouple the screen grids, with a new discrete decoupling cap added to decouple the phase inverter stage. Because the screen grid decoupling cap is so small, no Screen Stability resistors were required in the development model, and are not shown in the pic provided. However, they ARE shown on the amplifier page, since any unit destined for regular service will almost certainly have the can cap replaced, with 40-50 uF being a good practical value to useat this point. With such a value, Screen Stability resistors will be required, andare therefore shown on the schematic.

Thanks for this great thread. I'm rebuilding a 93-04 I picked up at a garage sale.
Regarding the cap can replacement (described above), I'm confused about which caps in the power supply should be in the 40-50 uf range. All four? Some of them?

 

[RUmad]

Pretty sure Dave’s schematic called for, in order, 40-30-10-30. I just flipped mine over to see what I used and they are 40-40-22-22. And I would assume I made sure those values were acceptable with Dave and co. before I used them.

 

[gadget73]

I built one recently with the supply as 47-100-100-33-33. The buffer stage gets an extra filter. It worked out fine, though I perhaps should have made the first stage a 33 instead of the 47.

 

[rjpjnk]

I’ve been of the impression that only the first cap after the rectifier needs to be limited in value and as for the others, the bigger the better.