Steve -- If the signal level supplied from your 400 to the 1800 during your initial tests were such that the volume control on the 1800 seemed to operate in the normal range of expectations, then V10 is off the hook, as you might remember that it was included in the signal chain at that point. The portion of the audio chain not checked with the 1800 tests includes V11 through V17 -- but with all new tubes, the possibility of a tube being the problem seems unlikely. However, of those tubes, V11 serves BOTH channels, while V12 through V17 are all dedicated to one channel or the other.
Other things to check if the sound is weak: check that the tape monitor switch is not dirty, nor that the speaker on/off switch is as well.
Another way to test things is to do the previous tests with the 1800 somewhat in reverse. That is, come out of the recording output jacks of the 1800 using it as the source this time, and go into TAPE MONITOR inputs on the 400 with your interconnects. Turn the Tape Monitor Sw on the 400 ON. Now, every thing is reversed, with the 400 acting to control volume, tone, and balance of the sound, with the 1800 being the source. If there are problems in the 400 from V11 going forward, it will show itself with this test, with the same weak sound you noted earlier when it was operating in a complete stand alone fashion.
I hope this helps!
Dave
:thmbsp:
