The Fullerwell

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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crgfrench
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Re: The Fullerwell

Postby crgfrench » Sun 02/10/19 10:53 pm

Here are some things I was losing sleep over, it’s my first amp build. Probably worried too much; all went pretty well.

Firing up the Denim Deluxe, What could go wrong:

Circuit could be designed poorly.

I could have done some bad soldering.

I may have used improper spec components.

The components could be placed in poor locations causing hum or noise (transformers would likely be the main culprit).

Short in circuit somewhere, or an open circuit somewhere .

I may have installed a faulty component (even if new).

Any of the passive components’ value could be too far off from spec (carbon composite resistors most likely).

I could have misconnected wiring during construction.

The board or wire can’t handle load & fries.

Poor lead dress could lead to hum or noise.

The board bus could be laid out as an improper grounding scheme.

Negative Feedback could squeal due to being in phase. NFB squeal? Reconnect Output transformer primary wires (blue & brown) to pins 7 of the opposite power tube.

Did I interpret the socket pinouts upside down (pins 1-9 backward)? Seriously doubt this but hey I guess it’s possible.

Will my lightbulb current limiter work with the variac to protect the caps at startup?

Is the NOS Tesla rectifier tube OK?

Did I wire the 3 switched tone circuits (Boost, Blend & Bright) properly?

Did I wire the rectifier vintage/modern switch properly?

Did I wire the attenuator circuit properly?

Should the pot tabs be grounded?

Is it safe to run the high voltage B+ (340VDC) to the volume pot’s rotary switch?

Should the grid stoppers be on the sockets instead of the jacks?

Should I have twisted the pin 7 wires from the power tube plates to the OT?

Should I have used shielded cable instead of the red wires to V1?

Should have used modern quiet resistors in the chassis?

Does the capacitor draining resistor ruin the circuit?

Will the 580 AC Volts & 340 DC Volts kill me?
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crgfrench
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Re: The Fullerwell

Postby crgfrench » Fri 02/15/19 9:45 am

Quick update on the output tubes (all matched pairs, all with Mullard CV4004 in first preamp socket and JJ ECC823 in second preamp socket, with a NOS Tesla EZ81 recto):

1) JJ EL84 new production pair works well.

2) Bendix 6094 NOS pair works but oscillates at higher gain settings.

3) EH 6973 new production pair redplates and blue glow even at low gain settings.

So I plan to try a 250 Ohm 8W Brown Devil in place of the current 125 Ohm 8W to cool down the bias.

(For the 6094s and 6973s I am using noval-noval pin adapters made in China, that convert them each to the 6BQ5 pinout the sockets are wired for).
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crgfrench
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Re: The Fullerwell

Postby crgfrench » Tue 02/19/19 5:46 pm

Image So taking Josh’s advice, I decided to build #2 using a “real” board. As I posted earlier, the 5E3+ board from Tube Depot seems like a good candidate. I got most of the passives soldered last night, here is an image of it sitting on a catalogue. For this one I kept the 125 Ohm Brown Devil; changed to Sprague from TAD filter caps; switched from orange drop 715s to Mallory 150s; and switched from carbon composite to Dale milspec for most of the 1/2W resistors.
4B3C6CB5-5DC2-4860-804F-A724D5250B70.jpeg
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crgfrench
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Re: The Fullerwell

Postby crgfrench » Tue 02/19/19 6:00 pm

9786636F-B8CD-45CD-95CC-87978453E403.jpeg
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JMPGuitars
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Re: The Fullerwell

Postby JMPGuitars » Tue 02/19/19 8:46 pm

crgfrench wrote:
Sun 02/10/19 10:53 pm
Should the grid stoppers be on the sockets instead of the jacks?

Should I have twisted the pin 7 wires from the power tube plates to the OT?

Should I have used shielded cable instead of the red wires to V1?

Should have used modern quiet resistors in the chassis?

Does the capacitor draining resistor ruin the circuit?

Will the 580 AC Volts & 340 DC Volts kill me?
I thought I already replied to this. Oops. Anyway:

Yes, ideally the grid stoppers should be as close to the socket as possible. But if it's doing the job, don't overthink it.

The voltage level coming off the OT leads won't benefit from twisting them, but people often twist them anyway for aesthetic reasons.

If the "red wire" is your input wire, then yes, shielded wire is a good idea. If you're lucky enough to not be getting noise without the shielding, it's not a big deal.

I prefer modern resistors. Carbon comp resistors only benefit the tone in a couple places, and whether or not it has any perceivable effect is up for debate. I use up to 6 carbon comps in a build, that's it.

It's not the voltage that will call you; it's the current. ;) Play safe!

Thanks,
Josh
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