Tube Amps / Music Electronics
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|previous: Kevin F. He keeps telling me it's awesome. I... -- 4/22/1999 2:30 PM||View Thread|
|4/23/1999 7:30 AM|
|Doc||Re: Crate Stealth 50 watt|
Ok, I found the schematic and parts layout. Chect the chassis stickers to confirm this to be a model GT-50.
Contrary to our rather graying, fuzzy recollections in earlier posts, this amp is actually a fixed bias design, with adjustable grid bias voltage. Tube complement is (3) 12AX7 and a quad of 6V6GT.
To set the bias, locate a small screwdriver adjust pot mounted to the pcb near the middle of all the large power supply electrolytics (it's along the centerline of the board in line of sight with the Master volume controls). I don't know if the component ID numbers are marked beside each component, or under it where it's not visible, but I'll give component numbers to help you identify the locations. The bias pot is AP1. The voltage can be read at the junction of nearby resistors R94 and R95. At the end of the pcb are a lot of spade lug wiring terminals. The one farthest from the front panel is the main ground terminal (J25). That's where your other meter probe goes. The proper way to get stable meter readings is with an 8ohm resistive load across the output terminals, but if you don't have one make sure the speaker is connected.
The crate instructions say the correct bias voltage range is between -37v and -34v. Unfortunauely, our friends in St. Louis didn't send me the parts list and voltage readings chart, so I can't give you any plate voltage or screen voltage as references. Instructions also say that the input AC line current draw should not exceed 1 amp at idle. Use this for a check if you have a series ammeter connected to the line cord.
Lower values of negative grid bias (smaller negative number) cause more current to flow in the power tubes. I would try to keep the setting closer to the high number end of -37v for better tube life, but if it sounds too harsh when playing through it, move closer toward the -34v endpoint. The pot will allow voltage to be adjusted out of this safe range, so be careful & watch your meter reading. (There are no cathode standoff resistors to aid in idling current measurement.)
Some general info on this circuit may shed some light on why it might sound good, and why it doesn't quite. Ignoring all the solid state boost and tone altering circuitry, the clean channel is basically BF fender preamp with the classic tone stack between the first two triodes. The phase inverter is classic BF, complete w/ 100k & 82.5k plate resistors. It has a 20k/0.1uf presence control across a 4.7k tail resistor, 100k NFB off the speaker jack terminal. Actually, if you would like a cleaner sound, a 12AT7 can be plugged into V3 socket (the one close to the power supply) like the typical fender.
The boost settings send the signal through a torturous path through a lot of semiconductors like a typical crate, but also through a direct coupled triode (V2) pair cathode follower tone control stack just like bassman/marshall to provide a beefy drive signal. The clean signal has to travel through no less than three (3) 1uf nonpolar electrolytics (C24,C31,C67) between the preamp (V1), reverb ICs, FX loop buffer ICs, and the phase inverter. If these three caps were replaced with polyester film caps, the clean sound would be quite a bit clearer, maybe closer to that of a deluxe reverb. The output stage is push pull parallel, with individual 470 ohm screen resistors and 1.5k grid stoppers.
Hope some of this rambling helps.
|Mark Cameron This amp was also designed by Lee J... -- 4/23/1999 11:22 PM|
Kevin F. Well, We replaced the power tubes w... -- 4/26/1999 8:54 AM