fishy wrote:You can switch between the two pretty quickly with just an adjustment on the rheo to get you in the ballpark. I actually liked the octals biased quite hot and then switching the el84s.
Any reason(s) against using cathode bias? Seems that would make the switch even easier.
Any reason(s) against using cathode bias? Seems that would make the switch even easier.
Nice build!
Thanks Zippy.
Both are actuallly cathode biased but instead of using seperate cathode setups, I used a 100ohm in series with the rheostat and you adjust the rheo accordingly. You can pretty much zero out the rheo and get the 84s singing. The octals takes a little longer but it is still very quick.
Originally, I had planned to have two seperate cathode setups but changed my mind. Of course, there is still time to change my mind again
Hey guys. I had let this amp sleep on the work bench for a long time, occasionally throwing in a new tweak idea here and there. I finally decided to give it one good attempt to dial it in and put it back in the cabinet and start using it.
I had installed a set of used old stock Tungsol 5881 power tubes that I had previously decided sounded pretty good. Over the past few months I had added trimmer pots on the preamp board to allow me to tweak the resistance values above and below the standard values given on the Rocket schematic.
I started at the right side of the board as you look at my picture. I ran the resistance up and down on each trimmer until I liked the sound, then moved to the next trimmer and did the same thing. I adjusted the sound with a preference for the richest harmonic content or the fullest sound. I didn't worry about keeping the amp clean sounding, although you could definitely hear it clean up as I dialed the trimmers toward one extreme or the other.
I did not make any measurements while I was doing this, just turned the trimmer until I liked the sound of it. I also did not go back and readjust any of the previous pots after I dialed in a pot... not necessarily because this was a good idea, it just sounded really good and I didn't want to mess it up.
When I was done, I measured voltages and resistances and have posted them on the attached photo.
The end result is that the amp is really fat and rich. It breaks up beautifully although never goes into fuzz out. I think the coolest part of it for me is that it can put out some great ringing feedback as the volume starts to come up... although maybe a bit too loud for my preferences. Tonight I added my Air-Brake to the mix and things got even better. I was able to keep the sweet tone and the ringing feedback but brought the volume down about 50% to my ears...
A fun experiment and a great sounding amp.
rj
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Last edited by RJ Guitars on Fri Jul 24, 2009 12:27 pm, edited 1 time in total.
Seems like the traffic on the Trainwreck pages is a little thin these days. I guess everybody is enjoying the great outdoors this summer, what's not to like about that?
I record a lot of stuff and seldom ever find any of it worth posting. Zippy loaned me some first rate mikes - a Sennheiser and an SM-57, but I think my preamp and cheapo computer sound card pretty well nullify any good the mikes are doing. All my recordings are a bit scooped out but this one captured a significant fraction of what the amp was doing. The actual sound I hear seemed slightly more clean. I don't know if that was my ears or the recording equipment.
Apologies to all you pros for the playing and the recording quality... dang was I even listening to the drummer? If you can get past that, it's a very cool amp.
Volume control at about 2 O'clock with an Airbrake taking about half of the load off of the speakers.
Questions and kind comments are welcome. I get an adequate number of rude comments right here in my own house... keeps me humble... Don't quit your day job etc..
Gibson ES-339 Stock pickups. Volume controls almost all the way up on both pickups. 2 x 10 open back cabinet
rj
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RJ,
Looks as though you have put some hours into this one. Did you put together a voltage chart after going through all of this. Might be interesting to review that and see how things have shifted.
I'm assuming you had the 2nd stage bypass cap (value?) switched in to help drive the octals a little better.
I think that the cathode resistor is going to be a dial to taste value, but I'll make a semi-educated guess that 300 Ohms is a decent starting place. From there I'd suggest you try it at the extremes (while watching your bias current) and see what your ears tell you.
If you can get one of the big rheostats like I used then you can make it something you adjust in real time. You could also look for one of those 500 Ohm power resistors with the sliding contact on the side. This won't give you the same dial it while you play feedback but still a good way to experiment with bias voltages.
I need to look and see what the bias was on the 5881's when I finished up but I recall that I am not pushing them very hard. I did use an output attenuator to make the tubes work a bit harder though.
Hey guys...
I have a rockster build in progress and I have a question... I want to implement separate adjustable cathode bias for each of the power tubes. I know that the cathode resistor will double when split as opposed to when it is shared... And that each cathode resistor will need its own bypass cap.
What value of caps should I use with the split if the original shared cap is a 220uf as in the rockster schematic?
... and these same principals apply when you parallel both sections of a preamp tube as well. As in the case of V1a in the rocket/rockster build... Going from separate (or individual) to parallel you would divide the resistance in half and double the capacitance. So 1.5k would become 750 ohms and 25uf would be changed to 50uf right. Right?
I know this is just basic ohms law...right, parallel/series resistors and caps... I guess when you don't use it you loose it.
Does this sound right guys?
