Results 1 to 7 of 7

Thread: VHT Special 12/20 RT Driver circuitry

  1. #1
    x11
    x11 is offline
    Junior Member
    Join Date
    Sep 2010
    Posts
    3

    VHT Special 12/20 RT Driver circuitry

    Hi,

    I do have an VHT Special 12/20 RT open at my desk. The problem is that it does not sound very good. The Watts-potmeter does function kind-of-ok at pentode-modus (it lowers the volume, but introduces distortion), and hardly does anything in triode-modus. Then, when I use the tremolo as well the tremolo introduces distortion to the sound every time it lowers the tremolo volume. I have started looking into the schematics and did some research on this forum and came to the conclusion that the schematics-testing-points-voltages are not in line with my findings, but that is a well known issue with this amps-schematics. Then, I measure a great difference in voltages (compared to the given test voltages in the schematic) at the driver stage. The driver stage is a cathode-following phase inverter, and according to the schematics I should measure about 121V at the grid (TP20), where I measure just about 11V. Then the cathodes of the Power tubes, should measure 29V, where I measure a maximum of 18V. i tried to include the original schematic as well as a redrawn part by hand with is slightly different to the original schematic and included the measured voltages.
    Any help would be appreciated!
    Attached Thumbnails Attached Thumbnails 1220rt.jpg   img_3021.jpg  

  2. #2
    Senior Member
    Join Date
    Feb 2009
    Posts
    318
    Quote Originally Posted by x11 View Post
    Hi,

    I measure a great difference in voltages (compared to the given test voltages in the schematic) at the driver stage. The driver stage is a cathode-following phase inverter, and according to the schematics I should measure about 121V at the grid (TP20), where I measure just about 11V. Then the cathodes of the Power tubes, should measure 29V, where I measure a maximum of 18V. i tried to include the original schematic as well as a redrawn part by hand with is slightly different to the original schematic and included the measured voltages.
    Any help would be appreciated!
    I think the voltages on that factory schematic are completely wrong. For example, B+3 at the power supply is shown as 218V, yet the plate/anode of V5A (in the factory schematic) is shown at 294V. It's not possible for the plate to be higher than the supply node feeding it. In particular, the grid voltage of at TP 20 of "121" is clearly wrong. There is no way it would be higher than the voltage at TP13.

    I think your problem is more likely to be in the output stage. A sine wave generator and an oscilloscope would be your friend here.

  3. #3
    x11
    x11 is offline
    Junior Member
    Join Date
    Sep 2010
    Posts
    3
    Thanks for the reply. I do have a sine wave oscillator and a oscilloscope set up. The thing is I do not know how the amp should sound and as far as I can see the 'principle' of the Watts and tremolo work fine. But then, the amp does not sound good when I use the Watts-function, especially in combination of the tremolo. So I was very happy to find a schematic with test points, so I had something to start from.
    When I use a pure sine input signal of 0,5Vtt and set the Volume 1/2 and the Tone 1/2 the signal is nice and clean up to the part where it runs into the power tubes (Grid (point 6 in stead of 5)). Note: With the Watts potmeter fully open. When I turn down the Watts-pot the signal is a little topped of at the upper part.
    When I measure the output (anode, point 3) or at the speaker output, the signal is topped of at both sides (most likely caused by clipping), when I turn down the Watts potmeter, the amplitude goes down and the clipping increases (!), the sine becomes a square. But testing it like this, the Watts potmeter seems to do its job well (except introducing these harmonics by clipping the signal.) So I lower my input signal so that the output signal is not clipped. Now, in triode mode the Watts-potmeter does hardly react, most likely because the input signal is to little. I asked VHT for some advise as well, but so far they did not reply.

  4. #4
    Senior Member
    Join Date
    Feb 2009
    Posts
    318
    I suspect it's working as designed, especially if it will put out its rated power into a load with the watts knob up. It's not obvious from the schematic where the cathodes of the power tubes are connected to, but this looks like a cathode biased amp. It also uses "bias vary" tremolo. I suspect that as you lower the B+ using the "watts" knob (This is a "VVR" type circuit) the bias for the tubes is too low. (Not enough current.) It seems like a marginal design & or your tone expectations for the amp are unrealistic. You might be able to add a switch to change the value of the cathode resistors to make it more appropriate for a lower B+ setting, but you'd have to be very cautious to not forget the switch in the wrong setting with the "watts" knob up.

  5. #5
    x11
    x11 is offline
    Junior Member
    Join Date
    Sep 2010
    Posts
    3
    Great Octal. Thanks for your reply, and you are probably right. Reducing the power by lowering the voltage and reducing the 'headroom' will lead to distortion. I find this is kind of strange way to lower the outputs power. The cathodes of the power tubes go directly to the depth 2P-6T rotary switch that will change the sound by changing the frequency response at the bias circuitry. Is the tremolo bias-vary? It is connected to the grid (point 6) of the power tubes. I assume it is pulling doen the input signal instead of the bias. Anyway thanks for pulling me through, I'll lower my expectations for this one. When not using the Watts knob, the amp sounds brilliant!

  6. #6
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    2,736
    I've repaired a few of these amps and modded one for harp use straight from the box. All of them distorted when the power was lowered using the watts control. I haven't taken the trouble to look at load lines, but lowering the plate and screen voltage probably shifts operation of the output tubes into non-linearity (hence more distortion).

    Interestingly all the amps suffered a blown control MOSFET (Q1) and the associated diode ZD1, resulting in the amp working but no output power control.

  7. #7
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    8,915
    when I turn down the Watts potmeter, the amplitude goes down and the clipping increases (!), the sine becomes a square.
    Reducing the power by lowering the voltage and reducing the 'headroom' will lead to distortion.
    Thatīs exactly what it should do,lower the maximum power so it clips earlier.
    If you want to reduce olume without adding clipping yu just use the normal volume control.

    As of TP20 voltages: that schematic is mess , and you simply do NOT measure grid voltage to ground in such inverters, in fact similar Fender amps carry a small label or note: "do not measure" for the very good reason that it is a high voltage ultra high impedance point and meter/scope internal resistance messes with measurement big way.
    What you do is measure cathode voltage and assume grid is 1 or 2V lower than that.

    You can never ever have 60V cathode grid voltage difference in a 12AX7, even less grid more positive than cathode.
    And as mentioned above, plate voltage will never ever be higher than supply voltage.

    Whatb you measured on cathode and plate sound very reasonable.

    From what I see, I donīt like that amp design, very kludgy and contradicting , but, hey, we are not here to "redesign" but bto "repair", so thatīs beyond our scope.

    Just make it certain it puts out roughly what it states, meaning reaching 80% of advertised power is fine, Marketing Dept. always stretches things a bit and send it back to customer.

    Tremolo cutting off part of the cycle means tremolo signal is too strong, bias tremolo does exactly that: attenuate by driving power tubes into cutoff or near that, just donīt drive them beyond cutoff or signal will be chopped, of course, set depth a little lower or, worst case if customer complains "it should not do that" add some resistance in series with depth pot, try 47k, it might be enough .
    Juan Manuel Fahey

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. VHT 12/20 RT Reverb Dwell
    By Jazz Hands in forum Guitar Amps
    Replies: 3
    Last Post: 12-14-2016, 12:27 PM
  2. VHT Special 12/20RT low preamp voltages
    By audiopete in forum Maintenance, Troubleshooting & Repair
    Replies: 22
    Last Post: 03-24-2015, 10:01 PM
  3. 6l6 in VHT Special 6
    By kev1n in forum Mods & Tweaks
    Replies: 94
    Last Post: 09-16-2014, 03:11 PM
  4. Boss Roland RT-20 rotary ensemble need schematic
    By RMD in forum Schematic Requests
    Replies: 3
    Last Post: 09-15-2014, 02:39 AM
  5. VHT special 6 repair
    By jtlee75 in forum Maintenance, Troubleshooting & Repair
    Replies: 13
    Last Post: 07-03-2011, 08:47 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •