Marshall NMV Choke

[Littlewyan]

In a Marshall NMV Amp has anyone ever replaced the Choke with a 1K Resistor? Reason I ask is I really want to build a 4 hole Marshall in future but have no need for a 2 x EL34 50W Blaster. So I plan to use 2 x 6V6s running at 350V which should be 25Watts (Possibly 20W) Output with a LarMar Master Volume or an attenuator. To keep cost down I might not bother with a choke and just use a 1K Resistor, which would also protect the 6V6's Screen Grids a bit more.

Edit: Before anyone says anything, yes I know there is sod all difference between 50W and 25W, 25W is just a bit easier on the Attenuator Heat Wise and it will still make a bit of a difference.

[Blackburn]

Using a 1K instead of a choke is just like an Express. Add 1K screen resistors too for a good reason. I wouldn't bother with a choke if you're using a SS rect. And you're a bit high on expected output power running those 6V6s at 350v. In an Express, it's noted as a 22w amp with 6V6s and that's running them at over 400v.

My $.02

[Littlewyan]

Well the less output the better (not very often you hear that!), although I definitely won't notice a difference between 25w and 22w. I'll definitely be using 1K Screen Grid Resistors as well as the 1K Dropping Resistor. Should end up with a slightly softer tone I think due to the extra Screen sag. I know its not a real Marshall 4 Holer but its as close as I can get. Like I said I have no need for a 2 x EL34 50W Blaster.

[Phil_S]

If you are discussing a choke input design with a pair of 6V6, you are probably at or under 100mA. A 5H 100mA choke shouldn't set you back more than about $12. Example here: http://www.bluebellaudio.com/transchokes.htm Model 158M or 158Q (seems to be Hammond) or the Weber 9H 120ma https://taweber.powweb.com/store/magnetic.htm will set you back $12 plus shipping. Go with the choke!

[rooster]

Phil_S, the bluebellaudio is UK and the prices are in pounds, eh? The weber choke is $25 and typically 100 ohms.

So here's a thought. Either use a normal 100ohm choke and use 2.2K screen resistors, or use a high wattage 1K resister (25watt) and 1K screen resistors. For reference, see the JCM900, 2.2k screen resistors, nothing new.

And here's another thing of note. The early Plexi Marshalls did not use screen resistors, just a 100ohm choke - and these amps are considered by many to possess some TONE. How could this work, you might ask? It's because the real deal Mullard EL34s didn't need screen resistors. Which means that today, if you are using a real Plexi, you can use new tubes and add the screen resistors, of spend $375 a pair for NOS Mullard EL34s and go sans screen resistors.

So.....where is the TONE? Is it in the choke, the screen resistors, or the tubes?

[roberto]

50mA are ok for this kind of amp. The 155H has 5H@50mA and costs £ 7.90.

Also take care of the maximum voltage rating (not indicated on that page), that is 400V for the 155H.

[Littlewyan]

Thanks for all of the responses guys. I hear ya Rooster about the early day Plexis. I guess the choke would provide additional filtering at certain frequencies, thus affecting the overall tone of the amp. A 32uF Capacitor may not always provide good filtering for higher frequencies (hence why you sometimes see them with a .1uF Cap), whereas the choke may give that extra bit of filtering. I think I'll stick with a choke, I'm already altering the Plexi recipe by using 6V6s and lower HT Voltage!

[Phil_S]

Sorry about Blue Bell

This choke is advertised as $12. https://taweber.powweb.com/store/022699sch.jpg

[roberto]

If you want the amp to compress more, you can use a RC to feed screens, increasing the R up to 10k (that is too much) and decreasing the C down to 10uF (that could be not enough, depending on the tubes).

[matt h]

(deleted)

[roberto]

He's talking about a choke between plates and screens.

[Phil_S]

I was thinking choke input. Reflecting on what I said, I think maybe it was a half-baked comment and I should not have said it. Choke input has other implications for lower plate voltage.

[Tone Lover]

Ive built 2 6V6 plexi,s still have one , love them both go with the choke I havnt tried the resister but the ones with the choke sound really good . Go early lower filtering if you want more sag playing blues and classic rock . Go the later higher filtering if you do metal with a dirtbox once in a while. I ended up keeping the higher filtered one because I do a little metal once in a while and my friend who is a great blues guy talked me out of the other one he loved the feel of it.For me the 6V6's were really nice the 50 watter was just to much.
Bill

[Roe]

most plexis has screen resistors, although some early 50w amps came without (or with 470R). The mullard el34s also need screen resistors with original voltages. I've personally seen xf2s die in a 50w plexi with 470R screen resistors

[Littlewyan]

Tone lover - Did you just put 6V6s into a standard 50W Circuit or did you change things to make it work? I'm lowering the HT on mine to 350V, Output T Impedance to 6.6K (option of 8K though) and lowering the filtering to 16/16, 32/32 and 32/32 (lower voltage doesn't need as high filtering).