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Ask Amp Man: Upgrades for Silvertone and Fender Hot Rod Amps

August 13, 2013


A vintage Silvertone ad: Groovy! Tremolo and reverb for $149. Photo courtesy of silvertoneworld.net

Hi Jeff,

I recently read your column describing how to mod a Silvertone 1484 amp head ["Tweaking a Silvertone 1484,"" October 2011]. I just found one of these amps and plan on recapping it soon.

You describe placing 0.47 µF and 2.2 µF caps in the circuit to mod each channel’s gain and tone, and also tell us to orient these caps so the polarity is correct. I'm confused—are you suggesting using electrolytic caps? I can't find any polarized 0.47 µF caps. I was thinking about using Orange Drops in this mod. Will that work?

As far as replacing the standby switch with a pot, does it operate like a master volume? In the amp's original configuration, do the power tubes get any cooler when the standby switch is activated? Or is it just a kill switch?

Also, the output transformer on my 1484 has two output leads to the speaker jacks. There is a yellow wire and a red/yellow wire. Do you know which one is the 8 ohm and which one is the 4 ohm wire?

Thanks,
Ryan Peiper

Hi Ryan,

Let me clear up your questions so you can complete your project. First, regarding the cathode capacitor polarity: If a capacitor in this position is polarized, the positive end should always be connected toward the tube’s cathode, with negative connected toward ground. But a cap as small as a 0.47 µF usually isn’t polarized, and an Orange Drop would be fine here. A larger capacitor like a 2.2 µF is usually polarized, but it can also be sourced in an NP (“non-polarized”) version, in which case the orientation isn’t a concern.

When you replace the standby switch with a pot, yes, it functions as a type of master volume. This amp was not designed with substantial gain in the preamp section, so don’t expect the sonic assault of a JCM800. But you may achieve a bit more overdrive at lower volume levels, especially if you hit the front end hard with some type of boost pedal. And no, the amp does not operate any cooler with the existing standby switch engaged, as that doesn’t prevent high voltage from being applied to the output tubes. A better name for this switch might be “mute.”

Which of the output transformer secondary leads is the 8 ohm tap, and which is the 4 ohm? Short answer: There is no 8 ohm tap. This incredibly undersized transformer was supposedly used for both the 1484 and 1485 model Silvertones. The 1484 has an approximately 25-watt output with two 6L6 output tubes and two 12" speakers. The 1485 has twice the power, with four 6L6s and six 10" speakers! The 1484 uses one output transformer, and the yellow lead powers the 4 ohm load of the two 12" speakers. The 1485 uses two output transformers. The primary of each connects to only one pair of output tubes, and the red/yellow wire connects to three of the six 10" speakers for a 2.66-ohm load. It’s a unique design.

Hi Jeff,

Thanks for all the great amp info, particularly the articles on Fender’s Hot Rod series. I have a Hot Rod Deluxe (HRD) and an early silverface Bandmaster Reverb.

I have several questions about your HRD column. The first concerns the volume fluctuation issue. I read that the fluctuation could be due to failing plastic input jacks, supposedly a common issue for HRDs. I bought two metal Switchcraft jacks with non-conductive washers to replace the stock jacks, but I haven’t installed them yet. Also, I must have an earlier version of the HRD that doesn’t have an effects loop. (It’s type PR246, made in Mexico.) Is it still more likely that the two power resistors on the low voltage side are causing the volume fluctuation? Would using 10-watt resistors make any difference in terms of heat dissipation?

I also bought an audio taper volume pot to replace the existing linear taper one, which is supposedly the cause of the “from nothing to ear splitting” leap between volume settings of 1 and 3. Would that be a better fix than going the Hot Plate route?

Best Regards,
Eric

Hi Eric,

The HRD seems to be a very popular topic, probably because the HR line was one of Fender’s biggest sellers. By the way, your model PR246 should have the same front-panel loop as the other HRDs, although Fender likes to call it Pre Amp Out and Power Amp In.

The input jacks do fail quite often, typically due to a broken solder joint or pin because the jack nut comes loose. You can diagnose that failure by simply wiggling the jack while the guitar cable is plugged in. If this causes severe volume drops, replace the jack.

Replacing the problematic 5-watt resistors with 10-watt resistors doesn’t alter the amount of power flowing through the resistors, but the larger resistor should dissipate the power a bit better, so there may be some heat reduction.

Finally, installing a 100k audio taper pot in place of the stock 100k linear taper (or B-style) master volume pot should definitely give you more control. I recommend installing all these upgrades at the same time. The less often you pull the circuit board from one of these amps, the better—for you and the amp!

Warning: All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.