How do Caps & Resistors Affect Tone? | Magnatone 108 Vintage Amp Repair Pt. 1

Let's take a look at my first ever amp project, and see if we can't upgrade the tone!
00:00 - Magnatone 108 intro
02:47 - The guts!
07:44 - Baseline bias
09:40 - Tone demo 1
10:43 - FUZZ??
12:56 - Jerry rigging the bias
16:30 - Play Test with new bias
18:07 - Thoughts?
19:26 - Let's tackle the preamp gain & tone
23:16 - Tone change with 25uF Cathode Bypass cap
25:24 - Thoughts?
27:04 - Let's do a side by side comparison!
27:18 - 1.5k resistor + 25uF bypass cap
28:35 - Fascinating! What did you think?
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[@YeatzeeGuitar]

Blast from the past opening this old amp up! Going to enjoy tinkering with it over the next couple of videos 😀

[@dyamariv3628]

Woaahh! I had the same nerdy excited reaction as you! lol Never heard that done before, kinda mindblowingly cool! 😎Definitely 25uf and the 2.2k from the video sounds the best. Not so overly bloated/saturated but still has the punch. Looking forward to the next videos of this! 🤓

[@alaricmusic]

Awesome video. I own that amp too. The cap/resistor comparisons are awesome, very useful in general for this amp and any other amp.

[@Lu_Woods]

This will be fun. I have a Maggie Leilani that needs to be brought to standards.

[@ali2ndmail]

This amp sounded awesome with any of the overdriven settings! That fuzzy tone was reminiscent of Neil Young circa Cortez the Killer. Maybe the deluxe cab simulation helped a little with that.

[@YeatzeeGuitar]

For sure, it definitely sounds better through the virtual cab vs the alnico gold loaded cab right now. I think I'm going to toss the weber back in and see how I think it sounds

[@goodun2974]

The Stancor PC 8403 transformer (see specs listed below) is in my 1959- 1960 catalog; the next earliest catalog I have is for 1948 and it's not shown in there, but apparently it was manufactured into the early 60s because it's listed in several of my more recent Stancor catalogs. Specs are: 117 VAc input = 250-0-250 VAC, 90 MA DC; rectifier 5V 2A; 6.3V 2.5 A. Provides approximately 260 V DC with a capacitor input filter.

[@YeatzeeGuitar]

Does your catelog list the OT? Curious what impedence it looks for

[@MarkEagleton]

I like the 2.2k better, too. It was a little smoother sounding to my ears. I’m looking forward to hearing it in the cab and seeing what speaker you end up going with. REALLY cool sustain on those licks at 25:00.

[@goodun2974]

The Stancor A3824 is shown in my 1949 Stancor catalog: Single or push-pull plates 6k to 10k Ω, maximum primary DC 75 milliamps; 8 W audio output. Secondary impedance taps: 1, 2, and 4 Ω. Cost was $4.10 in 1949; it is still listed in the 1969 Stancor
catalog....

[@YeatzeeGuitar]

Wow 1ohm and 2 ohm options, never seen that before! Cool, I see it says 1 2 4 on it but I just doubted it'd have those as the impedance options.

[@goodun2974]

​@@YeatzeeGuitar , I looked through my catalogs a bit more and updated the post above. Anyway, if the speaker in the Magnatone isn't 4 ohms you should change it for best results. Or perhaps you should change your little plastic speaker jackbox by adding the 2 ohm tap with 2 jacks on it, so that you can use the internal speaker in the combo cab simultaneously with
another external 4 ohm speaker, In parallel, naturally.

[@goodun2974]

@@YeatzeeGuitar , Stancor catalogues also included a chart where you could look up the type of output tube and find a good matching transformer, and the one in your amp is indeed what they recommend for use with single 6V6 or a push-pull pair.

[@YeatzeeGuitar]

@@goodun2974 Cool, love the detail thank you man!

[@ran-diy-audio]

Awsome simple Amp, It made me want to build one, if you don't mind, could you give me the schematic? Beyond everything, the content is great and I hope you continue making videos about this interesting little amp. greetings from Argentina!

[@jjsant3250]

Looks like the 6V6 is very tired needing such a low cathode resistor at that plate voltage. Or the screen voltage is too low. I show a 5E1 (not 5F1) Champ had 305 plate volts with a 470 cathode resistor and 19 cathode volts which is ~11 plate watts = almost 100%.

[@YeatzeeGuitar]

I'm think it's the transformer not putting out the voltage, which was replaced long before I got it. I tried a new JJ 6v6 and it actually biased cooler, I just cut that out of the video for the sake of time.

[@audiotechlabs4650]

Sorry to intervene, it’s my understanding to bias at 75 percent. 100 percent will greatly shorten the life of the output tube. I bias single ended 75 percent and push pull output tubes at 70 percent…that’s just me.

[@YeatzeeGuitar]

100% is fine for cathode biased amps at idle. Nothing stopping you from going cooler of course, if you like that sound.

[@nicksregor4208]

Is that JAN tube just really tired? That low resistance might way overbias a new tube if so

[@YeatzeeGuitar]

Could be! We'll see when I circle back to this project

[@oldasrocks9121]

Isn't 90% the target for Class A?
12AX7 plate and bias resistors are affected together, measure the plate to cathode voltage. 😊

[@YeatzeeGuitar]

No clue! First time biasing something like this

[@oldasrocks9121]

​@@YeatzeeGuitarWell one good thing is there's no crossover distortion in Class A so no worries there.
JJ rates their 6V6 at 14w but it's been 12w forever for all others. Class A ought to be 90% but folks push it, Fender certainly did which is why some tweed Champs sound like they might explode. Almost all Champs get 470 ohm bias resistors from the factory but some can be rebiased up to 1K, depends.
Check your grid voltage if you're into fine tuning. Check against the tube manual operating condition curves to find out where the tube is living.

[@dickjonesify]

25uF/2.2K 🔥
Cool video man. Neat little amp

[@YeatzeeGuitar]

Thanks Dan!