Balanced Phase Inverter or Not?

[Andyroid]

Does anyone here have first hand experience with balancing the phase inverter in a BC30 and hearing the difference before and after? I'm asking because I don't often get chance to really crank my BC30 and in the past I've noticed MASSIVE differences between how different valves behave as the BC30 PI. My 'balanced' GE 12AX7WA sounds awful when cranked with the stock 100k ohm a side PI, but my EH and JJ's sound fine! I was wondering if balancing the PI circuit by changing one side to 82k might fix this? The 'awful' GE sound I'm referring to is an early breakup and flat/fizzy distortion.

 

Cheers

[Andyroid]

Sorry, should have said the EH is fine as the PI but JJ/Tesla may introduce the ice-pick/missing bass problem. May need to double check that, I never did really get to the bottom of the ice-pick syndrome. All I know is it doesn't have it now! (I'm using EH for the PI)

[Dannyboyee82]

Just my inexperienced tuppence worth..

 

..the reason may be down to the valve as opposed to the circuit, when I switched out the valves in my BC 30 I was advised NOT to change the EHX PI because it is a very well balanced valve.. anything different would have a negative effect (this was the dude at Watford Valves, not some bloke down the pub by the way). I only switched out V1 and V2 on the recommendation of the Watford Valves dude.. that, for me, sorted out the fizzy overdrive, a problem that is common with the EHX 12AX7. With JJ 12AX7's in V1 and V2 it now sounds smooth and sweet. I'd just leave the PI alone if I were you.

 

With regard to cranking the amp, I'm having a master volume fitted to my BC 30 this week.. I'll find out the details of the mod and post it for you on here, I don't think it will be finished until a week from now.. I take my amps to some old dude locally that takes his time doing stuff, but produces great results. But I'll get the details up once it's done.

 

The ice pick syndrome is cured on my amp too.. it may be down to me finding the sweet spot on the EQ.. or it may be down to the valve swap I did, or both.. but it is now a serious tone monster, I love it. New valves are 2x Svetlana Winged C's, 2x JJ 12AX7 (V1, V2) and a Sovtek 5AR4.

 

The only thing that was missing - the master volume - now I will hopefully be able to get some nice saturation without drowning out the band and giving everyone within 30 feet of my amp tinnitus!

 

Oh, and I've got a HBE Germania on the way! I'll soon be sorted!

[Dannyboyee82]

Jefrs - I see you did the master volume mod.. did you experiment with different size pots or just stick a 250k straight in? How does it sound flat out? The reason I am doing this mod is because with a strat this amp has masses of headroom.. to get some nice saturation you need to have the volume way past halfway on the clean channel, not being a fan of the drive channel I have opted to put a master volume in to be able to drive the preamp really hard and dictate the overall volume with the master. What are your thoughts on the mod? I see you mentioned that it has altered the tone somewhat.. is that because the power stage isn't working so hard? Have you tried cranking the master and using the channel one volume for overall levels? I'd be interested to hear anything you have to say about this! Cheers.

[jefrs]

Jefrs - I see you did the master volume mod.. did you experiment with different size pots or just stick a 250k straight in? How does it sound flat out? The reason I am doing this mod is because with a strat this amp has masses of headroom.. to get some nice saturation you need to have the volume way past halfway on the clean channel, not being a fan of the drive channel I have opted to put a master volume in to be able to drive the preamp really hard and dictate the overall volume with the master. What are your thoughts on the mod? I see you mentioned that it has altered the tone somewhat.. is that because the power stage isn't working so hard? Have you tried cranking the master and using the channel one volume for overall levels? I'd be interested to hear anything you have to say about this! Cheers.

 

Did you reply to the wrong thread? - I posted some comments on this over on the big BC30 mods thread. The MV is before the PI. It allows you to crank either "channel" for gain, and control the volume on the MV (there is only one channel, with a "hot-rod" section that switches in). The BC30 is a bit of a loud beast. I did this MV mod to allow home practice at civilised levels but it is also useful on stage. I don't think the MV altered the tone much, but I did a few other mods that did do so. I'm fairly sure the MV is a full size Alpha linear 250k replacing the 220k fixed R26, I think first of all I fitted a linear 500k by mistake, that strangled the tone a bit. It is worth considering that most output valves do not clip or sag, at least the 6L6GC in the BC30 are not prone to do so. The EL84 is prone to clipping because it actually offers some significant voltage gain (most output valves are just current drivers). Sag only happens if a power supply is undernourished.

 

Regarding the PI - in the BC30 I am currently using one of Rocco's (Watfords) balanced Harma Retro-ECC83 Mullard-clones, slightly better sound to my ears than the EH. Still got those on the reverb.

OTOH I'm not that big a fan of balanced PI, an unbalanced PI is more "vintage" and can give a pleasant "beat" to loud sustained notes, which works with swing-jazz as well as heavy rock. Old amps did not get "balanced" bottles, they got whatever came off the shelf. Using an ECC83 will usually be pretty well balanced as the two valves are in the same bottle. An amp using e.g. a pair of ECL82, where the PI is a pair of triodes split across two separate bottles is inevitably difficult to balance, and it does the weird BFO beat thing nicely.

[mtheory]

The PI should be very strong, high gain, and balanced. On a fixed bias amp (not sure whether or not this amp is), swapping this tube will change your output tube bias, which is probably why you're hearing it sound different with different PI tubes. If you swap this tube, the output tubes should be re-biased (assuming the amp is fixed, rather than cathode biased).

[jefrs]

The BC30 is cathode biased. Swapping the PI valve won't affect it.

 

Indeed swapping the output valves for other octal valves won't affect it, because the cathode resistor is near enough in the bias requirement.

 

On mine the "balanced" PI valve is a "normal" gain valve (rather than a high gain type), but gain is mostly determined by the circuit: the ECC83 is a high gain valve, mu=100 (for the ECL82 mentioned before, the triode mu=70). On the PI it is usually arranged that it is biased in the middle of its voltage swing i.e. clean with headroom. If its bias were arranged hot or cold i.e. to clip, then it would clip on both clean and drive channels, which would not do.

 

This is a push-pull amp. In a hifi amp we would want the push to equal the pull, but this is a guitar amp. You do not have to use a balanced PI valve, a normal "unbalanced" ECC83 will be balance well enough and may actually sound better with guitar (perhaps worth mentioning that the BC30 is a Bassman clone and works rather well with a bass into a suitable speaker).

 

The BC30 has taps for fixed bias, but the SoCal50 is fixed bias and also has NFB. You could convert the BC30 but if you want a fixed bias and NFB version of the BC30, get a SoCal50.