Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Bub on September 03, 2011, 10:39:58 pm
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Hi Gent's
I got my hands on a Tektronix 2213 scope, HP 3311A generator, and a HP 5382A frequency counter. I read the 2213 manual and followed calibration proceeders to the best of my ability, there's the catch. I think I have it set correctly. I'm testing my 5F6-A build. Channel 1, HI and LO jack seem to mirror the input coming from the signal generator set at about 500 Hz, Photo 2297 is input signal to amp. Photo 2298 is output from amp on channel 1, volume at about 7, any more than that I go into cut-off and current limiting almost equally so I think I am close to centre bias on V1. Channel 2 Lo jack is photo 2299 at about volume 5 and photo 2300 is Hi jack at about volume 2.5. I don't know if I have a problem or not, both channels sound good. I installed a bridge switch to join channel 1 and 2 together and when I join both triodes it looks like photo 2298, nice and clean. Am I looking for a problem on channel 2?
Thanks for any help
Rob
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What is the sig gen output level set to? 200mVpp is a typical input level for most guitar amps.
Are you using a scope probe? With a ground clip connected to amp chassis? If you are using a shielded cable with BNC on one end and gator clips on the other, be sure to connect the shield clip to chassis.
Where exactly on the schematic do you have the scope connected for each pic?
It's important to state the voltage level (Vpp) measured in each pic.
If the first pic is at the input jack of the amp and the second pic is at the speaker jack, then I'd say that looks right. The noise and distortion on the last 2 pics could simply be due to a probe connection problem in a high impedance circuit, especially if you don't use a ground clip. The sine wave should look pretty much like the input (only bigger) all the way to the power tube grid if you are not overdriving any stages.
I like to put a Tee connector on the sig gen and connect one cable to scope channel 1. Trigger scope on channel 1. Connect another cable to the Tee and the amp input jack. Set scope vertical mode to alternate and use channel 2 to probe the amp. This way you can walk thru the amp and immediately see signal gain, phase inversions, distortions, etc. and compare to the input signal.
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Hi sluckey
Photo 2297 is signal generator teed to scope channel 1. Channel 1 volts/div is set at .1 1X. I counted up 5 divisions for the top of the sign wave and 5 divisions below for the bottom when I set the amplitude of the signal generator. I checked output voltage with my Fluke and get 175 mV ac, in auto mode. Photo 2298, 2299, and 2300 are all scope channel 2 which is attached to my 4 ohm, 100 watt resistor. Volts/Div setting on scope channel 2 is 5 1X. Sec/Div is set at .5 ms. I tried a jumper wire to chassis ground with no effect.
In the next set of pics #2301 is scope on both. Signal generator to Lo input jack, amp Normal channel. I turned the volume to about 5 to super impose both patterns. I don't know but it looks good to me. Hi input jack of Normal channel volume at 3 to get the same picture. Pic 2302 is signal generator to Lo input jack, Bright channel. Volume is at about 2, anymore than that my pattern goes to pic 2303. The amp sounds great on both channels so I don't think there is anything wrong with the amp, just how I'm trying to get my pattern.
Thanks for the help
Rob