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Crate GT-100H Tube Biasing

Discussion in 'Amp Central Station' started by Flyers99, Nov 13, 2017.

  1. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    Hello all, new to the forum. I own a GT-100H Stealth Tube head and I tried searching for my amp on your forum but didn't find anything. I bought new EL34 tubes to put into my Crate GT-100H Stealth amp I've had since the mid 90's but I cannot find the bias adjustment on the circuit board. Anyone have any experience with these? I called Crate (now Loud Tech Inc I guess) and they couldn't help me. I was referred to a local service center and they couldn't (or wouldn't) give me any info either. Anyone know how to bias these tubes on this amp? I have uploaded the manual and schematic and they do not show the bias points. The manual says these can ONLY be done by an authorized service center. Anyone know the trick to biasing the tubes here? Any help is greatly appreciated.
     

    Attached Files:


  2. corliss1

    corliss1 Tele-Afflicted Gold Supporter

    Sep 13, 2008
    Lansing, MI
    That PDF is giving me errors when I try to open it and I'm not having much luck with a quick online search. Can you try to send that again or maybe take an image of the power tube section/power supply page?
     

  3. VintageSG

    VintageSG Tele-Afflicted Ad Free Member

    Mar 31, 2016
    Huddersfield, UK
    Measure at the junction of R75+R76 ( or R94+R95 ). Adjust using the trimmer pot AP1 located near R95 to give -45V.

    Buy a pair of bias probes ( you'll also need 3 multimeters ). Using the Robrob bias calculator, measure the B+ and set the relevant bias using AP1 ( preferred method. Well, mine anyway )

    Don't blow yourself up. Very high voltages are in there to bite you at the first opportunity.
     
    Thin69 and Wally like this.

  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    +1 on the danger aspect.
     

  5. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    Thank you all for the info!! I currently have a eurotube bias probe (single tube at a time, not 4 tubes at a time). Not sure where I'm going to get 2 more multimeters though. Will I have to have 3 of them connected at a time? Also, my new tubes are not a matched set. Will this cause issues with the adjustment? Sorry for all the questions, I'm new to this. I understand electronics and the dangers involved very well but I want to be able to bias my own amps as I have my 2 Mesa's waiting to be done as well. My Crate is not my main amp so its the best to experiment on for me.
     

  6. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    Also forgot to mention it had E34Ls Groove Tubes in it before hand. I don't know if these were original or not but it says on Rob's site that these tubes are 22W compared to regular EL34's which are 25W. The new tubes I bought also have all the pins in them. The old tubes are missing the pin at about 2 o'clock with the tube notch sitting at 6 o'clock so the configuration seems different between the two. Pin 8 is at about 11 o'clock on the old tube with the tube notch sitting at 6 o'clock. On the new tube pin 8 is sitting at about 5 o'clock with the tube notch sitting at 6 o'clock. Is this typical of tubes to be numbered differently on the base?
     

  7. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    I clicked on the document and it downloaded okay for me. Let me upload it again.
     

    Attached Files:


  8. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    If your bias probe reads current draw and plate voltage, that is all you need. If it does not read plate voltage, then you will need to read that voltage to get enough info to derive the plate dissipation. You will need only one meter for the job, ime.
    GT silkscreen logos on tubes from various manufacturer. What is the entire code? I don’t buy GT tubes and do not know what EL-34’s they offer....but in other tubes a ‘c’ at the end of the tube type indicates Chinese, R 8ndicates Russian and S indicates JJ ...Slovak. If you use 25 for the max plate dissipation, then you are safe no matter what EL-34 you have. If they are 22 watts MPD tubes, then using 25 will bias those 22cooler than if they were 25 watt MPD tubes.

    Your Mesa amps will not be adjustable via a trim pot. If they are fixed biased, then you will have to chNge a bias resistance or install a trim pot in order to adjust bias. This is how Mesa gets you to buy Mesa tubes. Fwiw, I have never installed a set of tubes in a fixed biased Mesa and seen higher current draw numbers. They are biased to the cool side.
     

  9. VintageSG

    VintageSG Tele-Afflicted Ad Free Member

    Mar 31, 2016
    Huddersfield, UK
    Using two bias probes at a time, with two meters, you can sort your valves out to a close pair of pairs. One pair in the 'pull', one pair in the 'push'. With one probe ( and one meter ), see which valve is running the hottest and use that one to set the bias. That way, you don't risk burning anything up. If you have a Dymo paper labeller, or a paint pen, mark the valve -bases- so you can identify which valve is which. Don't do a Ruby Tubes and stick paper labels to the glass. It may not catch fire, but it doesn't half smell as it chars and it is a little worrying when the whiff comes from your power stage. You could ask me how I know, or you could guess.

    The third meter is to monitor the B+. As the current the valves draw alters during the bias setting, the voltage will alter, which in turn influences the required setting. Halfway decent meters don't cost a lot and are very useful to have knocking around. While your at it, invest in a spare pair of leads and a pair of -insulated- croc clips. Replace the pointy ends of the leads with the croc clips. Now you're not doing a balancing act trying to keep a meter lead tip in contact with high voltages or wedging a lead under a ground tag in the hopes it will stay there ( it won't, never does ). Croc clips make it easy to work on the amp one handed. One handed is good. One handed is safe. Less risk of a tingle going across the chest.
     

  10. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    Here are pictures of the new (on the left) and old (on the right) tubes.
     

    Attached Files:


  11. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    Thanks for the advice, I guess I have some more purchases to make if I want to be accurate at biasing.
     

  12. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    That GT is a JJ El-34. Ime, it is a strong tube. 25 watts max plate dissipation.
     

  13. Flyers99

    Flyers99 TDPRI Member

    Age:
    49
    7
    Nov 13, 2017
    Cleveland OH
    The GT tube is one of the old ones. Is there any way to tell if the tube is still strong? Would I have to put it back into the circuit and check the bias voltage? I do not have a tube tester but I am thinking of getting one. Although, it seems to me that the tube amp is dying out as I have seen many guitar players using virtual racks instead of half stacks or combos. I am a half stack guy myself and prefer a head and 4 12's over a combo amp or rack set up.
     

  14. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The picture comes up sideways and I got confused as to what is new and old. The Valley Art tube is a Chinese tube. I have no experience with that tube....and probably never will? But...Imwould treat it as a tube that has 25 maximum plate dissipation.
    It may be that your lack of experience precludes your djusting the bias. There is a bias adjustment pot shown in the schematic. A person with some experience would find it easily. However, this entails pulling the chassis and understanding safety precautions and some basic understanding of amp circuits. IMHO, it is tech time for your amp.
    This is one of the reasons why the people at Crate and that service center hesitated to give you guidance. They do not want to be responsible for any harm that would come to a person who should not be inside that dngerous environment....a live amp circuit.
     

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