5F2A - New Build Low Volume - Input?

[goangling]

I am stuck - This is my 2nd 5F2A build using the Hoffman turret board - and layout.  First one perfect - this one low volume -  sounds great but it is very low on both instrument inputs.

Using the Harvard 6G10 schematic to look at expected voltages - I get the following values:
6V6  plate 358v
6V6 screen 333v
6V6 cathode 21v

12AX7 Plate voltage supply  289v
          Plate voltage dropped (pin 6) 180v  (pin 1) 198
          pin 8 = 1.5v, pin 3 = 1.3v

All look within range to me -  I'm looking at how I wired the input jacks - and I'm pretty sure i screwed that up -see pic

I've swapped tubes, swapped speakers, tried with guitar (all working on 2nd amp), traced layout, and double checked resistor and cap values.

Thanks for you help in advance.

[Stuff4bikes]

Did you wire the first amp's inputs like these?

[Latole]

IMO J2 on picture have wrong wiring

[sluckey]

Input jack wiring looks correct to me.

[goangling]

Thanks for looking - It is not wired like my previous amp as I have made a custom 2 sided aluminum chassis to fit into a vintage Jensen speaker cabinet that I picked up at an auction.  It was such a short jog to V1-2 i just went to the pin with the 68K resistors.  I have since tested and verified wiring as correct.  To verify correct wiring on the input (Rob Robinette) https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm   https://robrobinette.com/images/Guitar/Other/Dtar_Input_Resistance_Test.jpg

[goangling]

So I think I may have found the issue -  I am using a Classic Tone 40-18030 OT -  I desoldered and took some measurements - I put 9.95 V AC into the secondary black/green labeled as 8 ohm.  I get 248 V on the 8K ohm tap red/blue primaries for a winding/turns ratio of ~25:1- So I'm running a single 6V6 output using 8500 ohm plate load resistance.  Calculating the impedance ratio I get 621.  so 8500/621 = 13.7 ohm!?

Vin = 9.95
Vout =248
Ratio = 24.9
ImpRatio = 621

If I go backwards - an 8 ohm speaker, with the above measured values, would be "looking" for ~ 5000 ohm input - when it should be close to the 8000 ohms listed on the data sheet from Magnetic Components.

Does this sound right?

I've now got to pull the turret board and measure the other taps to get the whole picture...will do this tomorrow.. - Could they have the colors wrong on the taps? 

[mresistor]

- Could they have the colors wrong on the taps?

I sure hope not because I have used a few of those OTs..   They seem fine to me. I used them in Champs and Vibro Champs. I've got one on the bench right now and it's LOUD>

[sluckey]

Your test and calculations are right and they indicate the red and blue wires are indeed 5000Ω. So it would seem the wiring diagram is wrong. However, this is unlikely to be your problem.

I suggest you post voltage readings for all filter caps and all tube pins, including zero volt readings and filament readings. Some hi-rez pics of your built would be helpful too.

[goangling]

Well - I think this new OT is junk - It might work for a 5K ohm input with an 8 ohm speaker - but useless for what I bought it for-  I've attached all the values.  I'll have to get a new one - once I get that I'll see how it sounds and test and re-post voltages.

[sluckey]

You have two amps. Why not just swap the OTs?

[PRR]

> it is very low on both instrument inputs.

8k vs 5k is not why "it is very low on both instrument inputs." That would be just-audible.

And why not switch the primary leads? It IS possible the worker got interrupted and put them backward.

And why don't you measure (maybe post) voltages real soon now? Before you ship 5 pounds of iron around the world?

[goangling]

The voltages were posted on 1st post

6V6  plate 358v
6V6 screen 333v
6V6 cathode 21v

12AX7 Plate voltage supply  289v
          Plate voltage dropped (pin 6) 180v  (pin 1) 198
          pin 8 = 1.5v, pin 3 = 1.3v

 The heaters all check out at 6.7 V

@ Sluckey - I don't want to take apart a good working amp (1st build) - I'm not in that much of a hurry - I do have a Hammond 125SE that I ordered for another project that has shipped.

[goangling]

So I figured out the problem -  It was the shielded wire going from volume pot to pin 7 of the 12AX7.  I removed and soldered in solid core wire and the volume is what I expected it to be.  This however has increased the noise and hum, so figuring that the reduced volume was caused from signal bleed to ground - I carefully rewired the mini-shielded cable (RG-174) with the exact same result - low volume again.  Could this be related to the shielded wire I am using, it's like size 26g (thin)? I have some thicker shielded wire that I'm going to solder in today.  See attached for voltages and pic of shielded cables side by side.

Thanks!

[Stuff4bikes]

Did you ground one end of the shielded cable?

How do your cable ends look. Stripped back far enough so the shield and signal wire arent touching.

[goangling]

yes grounded one end of shield at volume pot - other end going to 12AX7 cut clean and not grounded.  This is what I thought was the problem 1st time, but when I re-did it I was super careful, and even looked under magnifier to be certain there were no frayed shield that could touch anything   

[tubeswell]

Could this be related to the shielded wire I am using,

Seems like it. Need some better quality stuff

[Latole]

Working with shied wire, we have to be very careful to not melt main conductor insulation with too much heat.

[goangling]

After replacing shielded wire 3X - found the low volume is associated with tying the shielded ground to the ground buss on the volume pot (like it is shown on the 5F2A build layout).  When grounding the RG-174 shield directly to the chassis the volume is excellent.  So I've fixed the volume problem ... but don't know why it works when grounded to chassis, and does not work when grounded to buss wire on the pots? 

I'm a total noob - but I'm learning lots - and having fun!

Thanks for the help - this forum is excellent.

[PRR]

> why it works when grounded to chassis, and does not work when grounded to buss wire on the pots? 

Maybe the buss is not connected to chassis?

[Latole]

> why it works when grounded to chassis, and does not work when grounded to buss wire on the pots? 

Maybe the buss is not connected to chassis?

Right.

[goangling]

I'm pretty sure it is a ground loop issue.  Bus is grounded...but at multiple points - In the process of re-configuring the pre-amp ground bus - and have a couple questions.

I have 2 input jacks, a volume pot, and a tone pot.  power switch is not on the volume pot

For pre-amp ground bus -  I am thinking of grounding at one of the input jacks:

1.) Should both input jacks be ground to chassis?  or just one - if just one which one? (Hi or Low)
2.) Should I isolate the pots from the chassis, or is it OK to leave them grounded?
3.) I had the ground bus soldered to the back of each pot - but the pots were also grounded (no isolation washers).  I like it - cause it gave that bus some solidity -  if I isolate the pots, can I keep the bus soldered to the back of those pots?

thanks!  this thing is almost done - i'll post pics of the final project soon.

[bmccowan]

If you look over this recent thread https://el34world.com/Forum/index.php?topic=24681.msg266354#msg266354 I think you will find the info you are looking for. Most folks here do not solder to the backs of pots and ground the preamp bus at one spot, at or near the input.

[goangling]

Thanks -
Working on this - it's much clearer now - I just had everything going to one spot -  This will be easy to get done right.