The 6550 Experience

[jazz1]

Hi shakti

Check this schematic , basically you can wire Stand By switch as is in original SL , one choke MUST be installed for screen grids DC supply smoothing , further you have two option to connect DC supply for preamp stages,
First option is to connect on B+ 560VDC line via three series power resistors (18K+10K+10K) , and in this way you will get highest DC supply for preamp stages which in return will give you the cleanest final tone ,
Second option is to connect preamp DC supply from screen grids 280VDC supply line via single 1K2 power resistor , in this way you will get more distorted final tone ,
it is not bad solution to have third separate fuse only for 6550 screen grids supply line ,
also on proposed schematic check three black dots beside three separate main transformer secondaries which indicate that all three secondaries MUST be mutually connected `in phase` to get the minimal hum and noise from amp ,
hope that this will help for your specific amp mod .

Best Regards

Jazz1 ( formerly known as Jazz )

[shakti]

Thanks for the updated schematic, Jazz, makes a lot of sense! I doubt I'll add a separate fuse for the screen supply, but I have decided to use the choke on the screens. Then I can experiment with where to tap the preamp HV supply from - full B+ supply or half B+ supply and customize the response with various power resistors. The preamp board already has an 18k "built in" between the stock screens node and the PI node.

Just one last thing - I'm not sure if I understand about the phase thing with the PT secondaries...the bias supply has one leg for the ground and one for the bias winding. I wired this per the stock specs, can't remember the colour codes. But for the heaters and the HV secondaries - are you saying I might get less hum simply by swapping the wires over? I'm not getting a lot of hum, it is very quiet with the "Larry" grounding scheme I am using, but there is ghosting which I'd like to get rid of.

[jazz1]

shakti

I hope that your power transformer manufacturer have made correct color codes regarding to correct mutual phasing of three independent secondaries , this can be specially important in case if power transformer is built without electrostatic shield between primary and secondary side , personally before start wiring PSU I always check for correct secondaries phasing with O-scope ,since if secondaries mutually are connected `out of phase` that easy can be the reason of excesive generated hum&buzz ,
any way , you can always check for correct phasing situation in next way to , leave HV secondary connected as is and than first swap two wires from negative bias supply (B-)secondary , than turn the amp on and listen for hum&buzz , leave those two wires in position where the generated hum&buzz is minimal ,
than repeat the same procedure with two thick filaments wires (6,3VAC), again leave those two wires in position where the generated hum&buzz is minimal ,
final amp stability check is next , with all four inputs grounded ( no any input cable inserted) turn slowly all six control pots to maximum (10) and listen , if everything is OK from loudspeaker you will be able to hear only slight hiss generated from vacuum tubes and other passive elements but without of any sound of ultrasonic oscillations and without of any sound of hum&buzz,
about `ghosting tone` effect , maybe that ghost tone is sign of some unwanted AF/HF oscillations ,
one solution is to try to connect Rfb resistor on different output transformer secondary tap , try all three options , on 4 ohm , 8ohm ,and 16 ohm tap ,finally you can try the same but with different value of Rfb resistor to .
(Rfb resistor mean negative feedback loop resistor)

[shakti]

That's very interesting info! Generally my builds have been very quiet without any hum, but there is a tiny bit of buzz with one of my 50w builds, and perhaps this one too if I am really critical. Generally they are much quieter than vintage Marshalls though. The 69 and on Super Leads seem particularly prone to buzz for some reason.

In this particular amp I've got M e r r e n transformers, and if anything that guy knows transformers inside out. I'm sure the colour codes for the bias winding is correct, but with the heaters and the HV secondaries they are identical, so it's not really obvious which lead goes to which side of the rectifier, or which lead goes to which half of the tube heaters. Something to experiment with on a rainy day.

Can't wait to try out these mods! Alas, I will have to wait...no time until mid December at least.

[Roe]

thanks for the info Jazz1

[shakti]

I finally have the time to try this mod on my 69SL/"West Coast" clone.

Just one final question for Dave W: what would be your recommended voltage range for the PI/V3 tube plates? If I go with a very high (560ish) plate voltage and screens with half of that, I have to option of feeding the preamp either from the full B+ or half B+. I tend to like cleaner sounds though, especially with a Strat and I am aiming swuarely for a Band of Gypsys type tone. Dynamics is everything here.

[shakti]

A few quick remarks:

- I like this amp better the more headroom it has, but that means high voltage in the preamp almost more than anything. I am using 6550s, 3.4k primary OT impedance, shared V1 cathode, 12AT7 in V3, but otherwise stock.

- When I run it in its stock configuration, i.e. feeding the screens, PI and preamp just downstream from the output tube plates, I get very good results with just a little bit of input voltage mismatch, so I end up with 520-530V on the plates. PI voltage (before the 82k/100k) is about 370V. I am using 1k screen grid resistors, but ideally I should go a little higher. Especially if I go for a higher mismatch to bump the overall voltages a little more.

- I tried to rewire so that the screens and downstream PI and preamp were fed from the half B+ point, and with high enough input voltage mismatch so I got 560V on the output tube plates (and 280V to the screens). Of course, this is a very safe way to run the screens, but preamp voltages were just way too low and while the amp still had a "robust" tone it wasn't like opening the gates to heaven. I also discovered that when running like this, the output tubes needed *much* less negative bias voltage than in stock mode, even with the high plate voltage.

- all in all; I need to try this mode of operation with just the screens fed the half B+, and the PI/preamp fed from the full B+ so I can get the preamp headroom I want/need. This is a more invasive mod though, and I don't have my full parts stash on hand at the moment so I can't get that done until a few months. One question though; when running the screens from half B+ (280V) I don't suppose individual screen resistors are wanted/needed? Part of the advantage of that setup should be to have the screens supply as steady as possible, so I suppose any sag introduced by screen resistors would be undesirable?

[jazz1]

shakti

All you need to do now is to connect preamp stage via three series connected resistors 18K+10K+10K on B+560VDC line ,( I guess that you already have those stock 10K+10K power resistors instaled ,
actually you can experiment with various value instead of that suggested 18K/3W res.just to get that desired or optimum supply voltage for preamp stage(in your opinion that is +370V), start with 22K/3W and than you can gradually lower that value down to 12K/3W max. but not lower .

screen grids need some load resistors even if supplied from B+/2 line(+280V) just to suppress oscillation and overload condition , the lower the values the less THD will be generated from output power stage , but that value have to be NOT less than 100 ohm per each screen grid .

[shakti]

Thanks, jazz!

I think this is what I'll do, but it won't happen until I have all my stuff collected together.
I am starting to stray a bit from a stock amp, so I am thinking about a few other mods and switches. Most likely an R/T mod (cascaded) on a push/pull switch on the normal channel volume pot. There is a cool Youtube video of a Marshall Major with R/T mod which sounds phenomenal. As I am wiring this amp for maximum headroom and dynamics I think an R/T mod could sound very, very cool.

Might also experiment with a zero loss FX loop and PPIMV, but we'll see...

[Roe]

I prefer a flying resistor over 2k2 screen resistors. With 280v on the screens you can try smaller screen resistors though. Perhaps 470R?

With my 45/100s, I get a thicker tone a full breakup with the screens at higher voltages (515v at idle). The Radiospares deluxe choke reduces the screens voltage and current, giving a cleaner and crisper tone that is brighter at full breakup. On the latter amp, the screen voltage drops from 530v to 400v.

[shakti]

BTW, for all this tweaking and contemplating this and that, I made one small change which actually had as much effect as anything... I had come to the habit of using a high capacitance coiled guitar cord. I love this cable with all Gibsons particularly, and often with Strats particularly when going straight into overdriven amps.

However, when running through my BOG effects lineup (Octavio, fuzz, Castledine Supra-Vibe) as well as a couple of Lehle switchers for the signal routing, this high capacitance cable messes too much with the clarity and snap. It seems to clutter up the mids, particularly when I add the Uni-Vibe preamp. Instead of this cool, slightly brittle response from the Uni-Vibe preamp, it ends up slightly harsh and cluttered in the mids with too much mid boost. Simply by swapping that coiled cord for a low capacitance one, things cleared up a *lot*! With a shared cathode and no V2a boost cap and the other tweaks I've mentioned, running into a Fryette Power Station with Eventide H9 in the FX loop for reverb (tweaked a room algorithm for a "Fillmore East" preset) I have never been closer to that elusive BOG sound and response! Any further tweaking would actually be more in order to have the amp running as safely as possible, perhaps just a tiny, tiny bit more headroom.

[shakti]

And another "discovery", somewhat on the side of this discussion, but nevertheless...

I bought a Fryette Valvulator GP/DI preamp/desktop amp to use primarily as a headphone practice device, but it also covers a lot of other bases. It's basically a 1w stand-alone tube amp/preamp which can be used for headphone practice (with stereo FX loop), direct recording, as a DI amp with cab simulation, or into a guitar cab...OR it can be used as a preamp and with line out into a power amp. Along with the Fryette Power Station or similar tube power amp it makes for an extremely flexible setup. The Fryette Power Station is both a reactive load and a very clean sounding 2x6L6GC power amp + more.

I was pleasantly surprised, and at the same time slightly miffed, that I was able to simulate very closely a lot of the classic recorded guitar tones with this setup, an Eventide H9 in the FX loop, and into various cabs. The cabs were crucial though - plugged into a 4x12 equipped with 4x 1965 T562 alnicos, a bit of plate reverb simulation, and some careful tweaking of the GP/DI, I got surprisingly close to a Bluesbreaker sound. With the same setup into a 4x12 with 1966 T1221s, it was *very* Fresh Cream. And with the right Strat and a T1281 cab, a different reverb sound and some more preamp tweaking, I could get very, very close to a Band of Gypsys sound.

This setup isn't going to replace any of my "real" amps, and as mentioned the cab was crucial in nailing those classic sounds, but as a low volume and extremely flexible and portable setup, it was hard to beat. Sounds and feels about 10000% better than the Axe-FX that I've tried, IMHO, and almost as flexible.

[daveweyer]

Something to keep in mind; the lowered screen voltage means the screens will not draw nearly as much current during full power situations, therefore the resistor on the screens has much less current limiting effect. Bottom line is that any size will work, and will lower the voltage at the screen grid only slightly at full output power. It is handy to have a resistor there to act as a fuse should a tube short, and even though the tendencies for oscillations are drastically lowered with the reduced screen voltage, it is probably wise to have a stopper there. Also remember that the voltage developed as AC across the screen resistor is current feedback, and lowers the gain of the tube accordingly. You could probably get by with 100 ohm resistors.
In situations where the plate voltage was only 580, I usually derived the preamp supply from the HV--this is a personal taste thing, but higher preamp supply voltages give a different distortion characteristic downstream.
I'll be sharing the split supply Marshall Major mod I just completed here on the forum in just a few days, maybe there will be more West Coast ideas that you can play with, possibly even some I haven't mentioned yet.
Cheers to the mods!

[shakti]

Still in the planning stages since I work away from my amps, stash and amp work area...will be able to finalize all the mods in about a month and a half.

I am currently using the amp in stock configuration except for shared V1 cathode, no V2a cathode bypass cap, a 12AT7 PI and 6550s. Plate voltage around 530. I am using a Fryette Power Station as a load and tube amp re-amper and adding reverb on the line level signal.

I am contemplating a post phase inverter master volume (PPIMV) and zero loss FX loop. If those two work well, I could use the Fryette for other duties. I don't need to cut a lot of volume, but looking to reduce by about 6-9 dB. Since I am trying to get this amp to play as cleanly as possible while still having some of that classic Marshall growl, do you think a PPIMV would work well? I still haven't made any amps with a PPIMV. Part of the idea With the West Coast mods is to have a great dynamic range from the power section but also the PI, and obviously a PPIMV will limit that somewhat. Re-amping like I do with the Fryette has the advantage of letting the main amp work exactly as it would under normal conditions, and getting the full dynamic range of the core amp, then re-amping for a more suitable volume level. However, that approach is not entirely without limitations; the core amp is being loaded by a reactive load and not a speaker, and even while dynamics from the core amp are well preserved, the re-amper has to be turned up somewhat to reproduce a somewhat realistic dynamic range.

Ultimately I need to try this myself, but I am curious if those of you who have tried a PPIMV have any viewpoints on this. A PPIMV does seem to work very well With a distorted preamp, but in this situation where I am looking for a cleaner and more dynamic sound, would it work well to reduce output level by 6-9 dB? Furthermore, is there any point in trying to bump the plate voltage further to the 560-80V range if I would be using it with a PPIMV?

[Tek465b]

Am using a PPIMV, i like it but i use it in conjunction with a hotplate connected in parallel(load setting) to my speaker(16||16=8Ohm).
Only way to get acceptable volume level in a bedroom with incredible tone for me .